^      MICROSCOPIC  STUDY  OF  THE  SILVER  ORES 
AND  THEIR  ASSOCIATED  MINERALS 

UC-NRLF 


A   DISSERTATION  

Submitted  to  the  Department  of  Geology 

AND  Mining  and  to  the  Committee  on  Graduate  Study 

OF  the  Leland  Stanford  Junior  University 

in  Partial  Fulfilment  of  the  Requirements  for 

the  Degree  of  Doctor  of  Philosophy 


B   M   SDD   MTM 


by 
F.  N.  GUILD 


'^A 


''^'ii^g^. 


IReprinted  from  Economic  Geology,  Vol.  Xll,  No.  4,  June,  1917.I 


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A  MICROSCOPIC  STUDY  OF  THE  SILVER  ORES 
AND  THEIR  ASSOCIATED  MINERALS 


A   DISSERTATION 

Submitted  to  the  Department  of  Geology 

AND  Mining  and  to  the  Committee  on  Graduate  Study 

OF  the  Leland  Stanford  Junior  University 

IN  Partial  Fulfilment  of  the  Requirements  for 

the  Degree  of  Doctor  of  Philosophy 


BY 

F.  N.  GUILD 


[Reprinted  from  Economic  Geology,  VoL  XII,  No.  4,  June,  191 7.] 


I  «    *  )  f 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 

LANCASTER,  HA. 

I9I7 


Press  of 

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A  MICROSCOPIC  STUDY  OF  THE  SILVER  ORES  AND 
THEIR  ASSOCIATED  MINERALS. 

F.  N.  GUILD. 

OUTLINE. 

Page. 

I.  Introduction     298 

II.  The  Characteristic  Early  Minerals  of  Silver  Deposits  299 

Pyrite,  Sphalerite  and  Arsenopyrite. 

III.  The  Early  Silver  Minerals  303 

Galena  (Argentiferous). 
Tetrahedrite  (Freibergite). 

IV.  The  Late  Silver  Minerals    309 

Stromeyerite.  Cerargyrite. 

Pyrargyrite,  Proustite.  Hunt'ilite. 

Stephanite.  Dyscrasite. 

Polybasite.  Brongniardite. 

Argentite.  Schirmerite. 

Silver. 

V.  The  Associated  Copper  Minerals  328 

Chalcopyrite. 

Bornite. 

Chalcocite,  Covellite. 

VI.  The  Associated  Nickel  and  Cobalt  Minerals  330 

VII.  The  Gangue  Minerals   333 

VIII.  The  Identification  of  Silver  Minerals  on  Polished  Surfaces  334 

IX.  Resume    338 

X.  Acknowledgments    340 

297 


3G6472 


Z98  F.  N.  GUILD. 

I.    INTRODUCTION. 

The  increasing  amount  of  space  devoted  by  many  of  the  jour- 
nals to  investigations  in  which  the  reflecting  microscope  is  em- 
ployed, is  sufficient  evidence  of  the  great  value  of  this  new  method 
of  attack  in  the  solution  of  problems  of  paragenesis  of  the  opaque 
minerals.  That  these  researches  are  confined  so  largely  to  the 
investigation  of  the  copper  sulphides  and  associated  minerals  is 
the  occasion  of  some  surprise.  Although  in  a  more  or  less  gen- 
eral way  it  has  been  appreciated  that  the  replacements  concerned 
in  ihe  process  of  copper  enrichment  are  common  phenomena  of 
nature,  yet  the  fact  that  these  processes  may,  in  their  complexity 
and  interest,  be  duplicated  in  the  deposition  of  silver  minerals, 
has  failed  to  receive  the  emphasis  it  deserved.  This  is  due  in  part 
to  the  greater  scarcity  of  silver  minerals  and  also  to  the  greater 
difficulty  involved  in  their  identification.  Moreover,  the  applica- 
tion of  these  methods  to  copper  deposits  has  been  a  significant 
factor  in  the  solution  of  economic  problems,  and  has  thus  acted 
as  a  stimulus  for  extended  researches  in  those  fields.  This  stage 
of  development  has  not  yet  been  realized  for  the  silver  ores. 

The  writer  became  impressed  with  the  attractive  features  of 
this  unexplored  region  while  investigating  certain  copper-silver 
ore  specimens  from  the  Silver  King  mine  near  Globe,  Arizona.  It 
was  therefore  decided  to  make  a  detailed  study  of  the  silver  ore 
minerals  with  the  hope  that  the  paragenetic  features  observed 
would  prove  as  useful  a  foundation  for  further  work  in  this  field 
as  has  been  the  epoch-making  paper  of  Graton  and  Murdoch  for 
copper  ores.^ 

The  number  of  minerals  which  can  be  determined  accurately 
on  polished  surfaces  by  the  reflecting  microscope  alone  is  exceed- 
ingly small.  This  is  particularly  appreciated  when  one  under- 
takes the  study  of  the  sulpharsenites  and  sulphantimonites  of 
lead,  copper,  and  silver.  In  this  connection,  however,  it  should 
be  remembered  that  the  simple  tests  of  determinative  mineralogy 
leave  much  to  be  desired  in  identifying  these  compounds,  espe- 

1  Graton  and  Alurdoch,  "The  Sulphide  Ores  of  Copper;  Some  Results  of 
Microscopic  Study,"  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  45,  p.  26,  1913. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  299 

cially  when  not  well  crystallized.  At  the  present  time  the  use  of 
the  reflecting  microscope  in  the  study  of  the  opaque  minerals  is 
confined  mainly  to  the  investigation  of  mineralogical  relations 
rather  than  identification.  While  many  of  the  features  discussed 
in  the  following  pages  relate  to^  micro-chemical  tests  and  other 
methods  of  identification,  the  main  object  of  the  paper  is  to 
discuss  the  principles  of  ore  deposition  as  revealed  on  polished 
specimens.  The  investigation  of  many  minerals  not  containing 
silver  will  be  included  since  they  often  play  an  important  role  in 
the  natural  history  of  the  silver  minerals. 

The  order  of  deposition  of  the  various  minerals  is  found  to  be 
surprisingly  constant,  the  later  ones  replacing  the  earlier  ones  in 
a  definite  scheme  rarely  deviated  from,  so  that  the  minerals  may 
well  be  described  with  reference  to  their  position  in  this  scheme 
rather  than  by  employing  the  ambiguous  terms  primary  and  sec- 
ondary. With  this  idea  in  mind  the  minerals  considered  in  this 
investigation  will  be  taken  up  in  the  manner  already  outlined. 

II.    THE   CHARACTERISTIC   EARLY    MINERALS   OF    SILVER   DEPOSITS. 
PYRITE,  SPHALERITE  AND  ARSENOPYRITE. 

Pyrite. — Pyrite  is  rarely  absent  from  silver  deposits,  and  ac- 
cording to  some  authors,  plays  an  important  part  in  the  later  en- 
richment of  the  silver  minerals.  Thus  from  geological  consid- 
erations, Weed^  has  observed  that  enrichment  has  not  taken  place 
in  the  absence  of  this  sulphide  although  the  other  conditions  were 
favorable.  Where  pyrite  was  present  abundant  rich  silver  min- 
erals have  made  their  appearance.  In  the  opinion  of  Cooke^  these 
assumptions  have  been  confirmed  by  his  laboratory  experiments 
on  the  solution  of  silver  minerals  in  dilute  sulphuric  acid  and  the 
iron  sulphates.  These  are  very  iniportant  considerations.  If  the 
presence  of  ferric  sulphate  resulting  from  the  oxidation  of  pyrite 
is  a  necessary  factor  in  the  deposition  of  the  rich  later  silver  min- 
erals, then  downward  percolating  solutions  have  accomplished 

-  Weed,  "  The  Enrichment  of  Gold  and  Silver  Veins,"  Trans.  Am.  Inst. 
Min.  Eng.,  Vol.  30,  p.  439,  1900. 

3  Cooke,  "  The  Secondary  Enrichment  of  Silver  Ores,"  Jour.  GeoL,  Vol. 
21,  p.  I,  1913. 


300  F.  N.  GUILD. 

the  feat,  and  these  minerals  are  definitely  established  as  super- 
gene.  While  there  seems  to  be  no  doubt  that  the  presence  of  iron 
salts  in  acid  solution  favors  the  solution  and  transportation  of 
many  sulphides,  it  has  by  no  means  been  proved  that  they  con- 
stitute the  only  solvent  for  the  rich  silver  minerals.  Hypogene 
solutions  are  the  chief  agents  in  the  deposition  of  vein  minerals 
and  that  fact  alone  is  sufficient  evidence  of  their  great  ability  as 
efficient  solvents. 

Pyrite  is  normally  the  first  sulphide  to  form  in  veins  of  the 
common  type.  It  may  be  preceded  by  pyrrhotite  and  arseno- 
pyrite  in  higher  temperature  deposits  but  is  followed  by  spha- 
lerite, tetrahedrite,  and  galena.  In  the  investigation  of  the  silver 
ores  the  appearance  of  later  generations  of  pyrite  has  rarely 
been  observed.  When  so  observed  it  may  be  found  in  one  of  two 
types :  First,  as  second  generation  hypogene  pyrite,  and  second, 
as  supergene  pyrite  (or  marcasite).  An  example  of  the  first  case 
has  been  noted  in  a  specimen  from  the  Silversmith  mine,  Sandon, 
B.  C,  in  which  a  thin  veinlet  of  pyrite  was  found  crossing 
sphalerite.  In  a  similar  manner  it  was  found  crossing  tetra- 
hedrite also  observed  in  only  one  specimen.  These  occurrences 
are  believed  to  represent  early  fractures  in  which  a  fresh  supply 
of  hypogene  solution  has  entered.  The  veinlets  represent  filling 
rather  than  active  replacement  and  are  therefore  evidences  of 
mechanical  readjustment  rather  than  a  new  chemical*  equilibrium. 
Supergene  pyrite,  however,  seems  to  be  the  result  of  definite  re- 
placement. This  has  been  observed  in  a  specimen  of  argentite 
from  Freiberg,  Saxony,  and  is  illustrated  in  Plate  XVI.,  C.  Here 
the  argentite  has  crystallized  in  cavities  and  the  replacement  by 
pyrite  (or  marcasite)  has  progressed  along  borders  in  the  form 
of  groups  of  spheroidal  masses  with  metacolloidal  structure. 
Hintze  also  notes  pyrite  as  pseudomorph  after  argentite  at 
Joachimsthal.'*  A  similar  alteration  of  polybasite  to  pyrite  (and 
pyrargyrite)  in  the  Neihart  district,  Montana,  has  been  observed 
by  Weed.^     These  occurrences  call  to  mind  the  experiments  of 

■*  Hintze,  "  Handbuch  der  Mineralogie,"  Bd.  I,  p.  444. 

^Weed,   "The  Enrichment  of  Gold  and  Silver  Veins,"   Trans.  Am.  Inst. 
Mill.  Eng.,  Vol.  30,  p.  446,  1900. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  3° I 

Allen  who  found  that  iron  disulphide  as  pyrite  and  marcasite  was 
deposited  from  a  cold  solution  of  low  acidity  containing  ferrous 
sulphate,  sulphur  and  hydrogen  sulphide.^  Pyrite  is  thought  to 
have  been  deposited  by  descending  surface  waters  in  the  George- 
town district,  Colorado,  and  is  therefore  referred  to  as  secondary 
pyriteJ 

In  silver  deposits  pyrite  is  often  found  to  suffer  the  changes 
so  characteristically  observed  in  the  copper  veins.  Borders  and 
veinlets  of  chalcopyrite  appear  which  are  later  replaced  by 
bornite,  chalcocite  and  covellite.  The  Silver  King  mine  in  Ari- 
zona, being  particularly  rich  in  copper,  shows  these  features  to  a 
remarkable  degree.  Here,  as  will  appear  in  later  descriptions,  all 
the  features  of  copper  enrichment  are  observed  parallel  with  in- 
teresting replacements  of  silver  minerals.  Stromeyerite  plays  the 
same  role  as  chalcocite  and  a  complex  series  of  replacements  is  to 
be  observed,  beginning  with  pyrite  and  ending  with  native  silver. 
In  deposits  where  copper  is  not  present  in  considerable  quantities, 
galena  is  found  to  be  the  most  frequent  replacing  mineral. 

Pyrite  in  association  with  the  silver  minerals  is  shown  in  Plate 
X,  A  and  B.  In  Fig.  A  pyrite  is  replaced  by  chalcopyrite,  which 
is  later  replaced  by  galena,  while  in  Fig.  B  galena  replaces  pyrite 
and  arsenopyrite  directly.  The  silver  minerals  have  come  in 
later  than  the  galena. 

Sphalerite. — Sphalerite  is  present  in  the  greater  number  of 
silver  ore  specimens  studied,  a  typical  example  appearing  in  Plate 
X.,  C.  It  is  not  found  actively  replacing  pyrite,  and  the  evidence 
of  its  later  age  rests  upon  mechanical  relations.  Thus  it  is  some- 
times observed  coming  in  after  a  brecciation  which  has  occurred 
later  than  the  deposition  of  pyrite.  It  is  commonly  filled  with 
minute  dots  of  chalcopyrite  arranged  in  rows  and  otherwise  crys- 
tallographically  distributed. 

The  occurrence  of  later  sphalerite  has  been  noted  by  Blow  at 
Leadville,  Colorado,  where  it  is  thought  to  have  been  dissolved  in 

6  Allen,  "  Sulphides  of  Iron  and  their  Genesis,"  Min.  and  Sci.  Press,  Vol. 

103,  p.  414,  iQii- 

^  Spurr,  Garrey  and  Ball,  "  Economic  Geology  of  the  Georgetown  Quad- 
rangle, Colorado,"  Prof.  Paper  U.  S.  Geol.  Surv.,  63,  p.  144,  1908. 


J 


02  F.  N.  GUILD. 


the  oxidized  zone  and  redeposited  below. ^  Emmons,  however, 
is  not  of  the  opinion  that  sphalerite  "  is  precipitated  as  a  secondary 
mineral  along  with  copper,  silver  and  gold."''  In  all  of  the  speci- 
mens studied  in  connection  with  this  investigation,  it  is  an  early 
hypogene  mineral  and  it  has  not  been  observed  replacing  any  of 
the  later  sulphides.  Minerals  found  replacing  sphalerite  are  tetra- 
hedrite,  galena,  chalcopyrite,  chalcocite,  covellite,  stromeyerite 
and  ruby  silver. 

Arsenopyrite. — Arsenopyrite  is  probably  a  high  temperature 
mineral  as  shown  by  its  presence  in  pegmatites,  contact  deposits 
and  magmatic  differentiates.  It  is  occasionally  present  in  silver 
veins  where  it  is  found  to  be  still  earlier  than  pyrite  ordinarily  the 
first  sulphide  to  be  deposited.  This  is  to  be  expected  since  its 
other  modes  of  occurrences  show  it  to  be  more  closely  connected 
with  the  magmatic  period  than  other  vein  sulphides.  In  associa- 
tion with  pyrite,  galena  and  proustite  it  is  shown  in  Plate  X.,  B,  a 
photomicrograph  of  a  polished  specimen  from  an  unknown 
locality  in  Mexico.  In  this  specimen  pyrite  does  not  appear  to  be 
actively  replacing  arsenopyrite,  but  its  later  deposition  is  inferred 
from  the  fact  that  the  arsenopyrite  is  greatly  fractured  while 
pyrite  is  still  intact.  Pyrite  was  deposited  later  than  the  frac- 
turing. The  corrosion  of  arsenopyrite  may  have  furnished  arsenic 
for  the  proustite  molecule. 

In  a  specimen  of  galena  containing  arsenopyrite  from  Rimini, 
Montana,  it  appeared  in  hand  specimens  as  later  than  galena  in 
the  form  of  tongues  and  veinlike  masses.  On  examination  with 
the  reflecting  microscope  the  veins  were  found  to  be  crossed  and 
broken  up  by  a  network  of  galena  veinlets  thus  showing  its 
residual  character.  This  peculiarity  has  also  been  obsei"ved  with 
reference  to  tetrahedrite.  The  microscope  sho\vs  this  mineral  to 
be  always  earlier  than  galena,  yet  in  hand  specimens  forms  are 
observed  that  might  lead  to  the  reverse  conclusion. 

8  Blow,  "  Geology  and  Ore  Deposits  of  Iron  Hill,  Leadville,  Colo.,"  Trans. 
Am.  Inst.  Min.  Eng.,  Vol.  18,  p.  171,  1889-1890. 

8  Emmons,  "The  Enrichment  of  Sulphide  Ores,"  Bui.  529,  U.  S.  Geol.  Surv., 
p.  143,  1913. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  303 

III.    THE  EARLY  SILVER  MINERALS. 

Argentiferous  Galena  and  Tetrahedrite. 

Galena. — Galena  with  a  varying  percentage  of  silver  is  one  of 
the  most  common  minerals  in  silver  deposits  in  which  it  occurs 
replacing  the  earher  sulphides  including  arsenopyrite,  pyrite, 
sphalerite,  tetrahedrite  and  chalcopyrite.  In  Plate  X.,  B,  it  is  seen 
replacing  both  pyrite  and  arsenopyrite.  In  Plate  X.,  D,  it  is  seen  to 
replace  tetrahedrite  and  chalcopyrite.  Pyrite  and  arsenopyrite  are 
not  replaced  by  it  in  case  the  later  minerals  mentioned  above  are 
also  present.  This  fixes  its  position  in  the  order  of  deposition 
quite  definitely  as  follows:  (i)  Arsenopyrite,  (2)  pyrite,  (3) 
sphalerite,  (4)  tetrahedrite,  (5)  chalcopyrite,  (6)  galena.  The 
rich  silver  minerals  are  still  later  as  will  appear  in  the  descrip- 
tions to  follow.  The  comparatively  late  appearance  of  galena 
might  lead  one  to  place  it  among  the  supergene  minerals.  Rarely, 
however,  does  it  seem  to  be  associated  genetically  with  oxidation 
processes  in  such  a  way  as  to  suggest  deposition  from  descending 
solutions.  Blow  found  veinlets  of  galena  penetrating  limestone 
directly  below  the  oxidized  ores.^°  Irving  and  Bancroft  state 
positively  that  a  portion  of  the  galena  at  Lake  City,  Colo.,  is 
connected  with  oxidation  processes.^ ^  The  great  insolubility  of 
galena  under  surface  conditions  has  been  well  pointed  out  by 
Emmons.  Its  occurrence  in  outcrops,  in  placers,  and  the  fact  that 
it  has  frequently  been  plowed  up  in  the  fields  of  the  Wisconsin 
zinc  district  are  convincing  evidences  of  this  property.^" 

Since  galena  is  considered  to  be  the  source  of  much  of  the  silver 
appearing  in  deposits  of  the  precious  metals,  it  is  a  matter  of 
great  interest  to  determine  if  possible  the  form  in  which  the  silver 
exists.  There  are  at  least  three  possibilities.  First,  the  silver 
molecule  usually  thought  to  be  argentite  may  be  present  as  a  solid 
solution  or  isomorphous  constituent ;  second,  it  may  occur  as  in- 

1°  Blow,  "  Geology  and  Ore  Deposits  of  Iron  Hill,  Leadville,  Colo.,"  Trans. 
Am.  Inst.  Mill.  Eng.,  Vol.  18,  p.  169,  1889-1890. 

11  Irving  and  Bancroft,  "  Geology  and  Ore  Deposits  near  Lake  City,  Colo.," 
Bui.  U.  S.  Geol.  Surv.,  No.  478,  p.  97,  191 1. 

12  Emmons,  "  The  Enrichment  of  Sulphide  Ores,"  Bui.  U.  S.  Geol.  Surv., 
No.  529,  p.  138,  1913. 


304  F.  N.  GUILD. 

elusions  or  microscopic  particles  of  definite  minerals;  and,  third, 
it  may  occur  in  the  form  of  submicroscopic  particles.  Nissen  and 
Hoyt  have  conducted  laboratory-  experiments  with  the  hope  of 
throwing  some  light  on  the  subject.^^  They  found  that  lead 
sulphide  when  fused  was  capable  of  absorbing  or  holding  in  solid 
solution  on  cooling  less  than  0.2  per  cent,  silver  sulphide.  The  ex- 
cess separated  out  in  the  form  of  definite  grains  of  argentite  as 
shown  by  their  photomicrographs.  These  experiments  were  not 
performed  under  the  same  conditions  of  temperature  as  existed 
in  the  veins  when  the  deposition  of  galena  took  place  and  there- 
fore are  not  conclusive.  It  is  quite  possible  that  very  different 
relationships  between  the  two  sulphides  may  exist  at  the  lower 
temperature  of  deposition  from  water  solution.  Nevertheless 
field  observations  have  strongly  suggested  conclusions  similar  to 
those  arrived  at  by  Nissen  and  Hoyt.  Thus  in  the  silver  deposits 
of  Bingham,  Utah,  Boutwell  found  those  ores  to  assay  highest  in 
silver  Vvhich  contained  the  largest  amount  of  some  other  silver 
mineral,  in  this  case  tetrahedrite  (freibergite).  Ranking  next  to 
these  were  the  "  black  sulphides "  thought  to  contain  tellurides 
and  last  of  all  the  pure  galena  samples.^'*  In  the  silver  deposits 
near  Lake  City,  Colo.,  Irving  and  Bancroft  report  that  "  little  of 
the  argentiferous  galena  is  rich  in  silver  in  any  of  these  districts 
unless  accompanied  by  tetrahedrite  or  some  rich  secondary  silver 
mineral."  Surfaces  were  polished  and  when  no  admixtures  were 
observed  the  assay  values  indicated  only  10  to  15  ounces  per  ton.^^ 
A  large  number  of  specimens  of  argentiferous  galena  from 
various  localities  was  investigated  by  a  comparative  study  of 
polished  surfaces  and  quantitative  determinations  and  many  in- 
structive features  observed.  On  etching  the  surface  with  nitric 
acid  or  hydrogen  peroxide,  all  galena  grains  showing  above  o.  10 
per  cent,  silver  revealed  an  appreciable  number  of  spots  of  either 
argentite  or  tetrahedrite,  frequently  both.    These  spots  were  fre- 

13  Nissen  and  Hoyt,  "Silver  in  Argentiferous  Galena  Ores,"  Econ.  Geol., 
Vol.  10,  p.  172,  1915. 

1*  Boutwell,  "  Economic  Geology  of  the  Bingham  Mining  District,"  Prof. 
Paper,  U.  S.  Geol.  Surv.,  No.  38,  p.  113,  1905. 

1^  Irving  and  Bancroft,  "  Geolcjgy  and  Ore  Deposits  near  Lake  City,  Colo.," 
Bui.  No.  478,  U.  S.  Geol.  Surv.,  p.  56,  1911. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  3° 5 

qnently  so  minute  that  even  when  present  in  great  abundance, 
the  percentage  of  silver  was  not  raised  above  0.39  per  cent.  Such 
a  specimen  from  Rimini,  Mont.,  is  illustrated  in  Plate  XL,  A.  In 
this  specimen  the  microscopic  inclusions  are  of  four  distinct  min- 
erals, three  of  which  could  be  identified.  Tetrahedrite  appeared 
harder  than  galena  and  could  be  recognized  by  its  relief.  More- 
over it  could  be  compared  with  other  grains  in  the  specimen  of 
sufficient  size  to  permit  of  micro-chemical  tests.  Argentite  ap- 
peared slightly  softer  than  galena  and  was  practically  invisible 
before  etching.  Ruby  silver  could  also  be  distinguished  by  its 
bluish  tint  as  compared  with  galena.  The  fourth  mineral,  also 
softer  than  galena,  may  have  been  polybasite,  stephanite  or  some 
of  the  sulfo-salts  of  lead  or  copper.  Only  when  the  spots  aip- 
peared  to  be  of  considerable  size,  in  fact  visible  with  a  hand  lens, 
did  the  percentage  reach  0.50.  These  begin  to  show  evidence  of 
a  later  addition  of  the  silver  mineral  with  the  exception  of  those 
containing  tetrahedrite.  This  mineral  then  has  the  characteristic 
appearance  of  residual  grains  left  over  in  the  replacement  of 
tetrahedrite  by  galena.  The  grains  become  more  numerous  or 
larger  as  tetrahedrite  areas  of  considerable  size  are  approached. 
The  later  entrance  of  the  rich  silver  minerals  is  illustrated  in 
Plate  XL,  ^,  representing  a  polished  specimen  from  Tonopah, 
Nev.,  and  Plate  XL,  C,  a  specimen  from  the  Reco  mine,  Sandon, 
B.  C.  In  Plate  XL,  B,  the  spots  are  proustite  and  are  associated 
with  thin  veinlets  of  the  same  mineral  not  shown  in  the  photo- 
graph. Identical  structures  have  been  observed  in  specimens  of 
galena  from  Freiberg,  Saxony,  which  contained  1.12  per  cent, 
silver.  Plate  XL,  C,  shows  pyrargyrite  following  cleavage  di- 
rections in  galena.  The  specimen,  as  assayed  by  the  methods 
described  below,  contained  0.54  per  cent,  silver. 

On  the  other  hand,  specimens  of  argentiferous  galena  showing 
on  etching  only  occasionally  spots  of  the  silver  minerals  were 
found  to  contain  less  than  o.io  per  cent,  silver.  Thus  galena 
from  the  Bunker  Hill  mine,  Coeur  d'Alene  district,  Idaho,  show- 
ing only  now  and  then  in  the  field  of  view  one  spot  contained  0.08 
per  cent,  silver.  Another  specimen  from  the  same  mine  sometimes 
with  three  spots  in  the  field  of  view  contained  0.09  per  cent,  silver. 


306  F.  N.  GUILD. 

A  specimen  from  the  Hercules  mine  showing  an  occasional  spot 
gave  0.108  per  cent,  silver.  Areas  in  the  same  hand  specimen  with 
25  to  30  spots  in  the  field  of  view  (No.  5,  Leitz  objective)  con- 
tained 0.24  per  cent,  silver.  These  spots  were  practically  all  iden- 
tified as  tetrahedrite.  A  specimen  of  galena  was  found  in  the 
Old  Yuma  mine,  Pima  Co.,  Arizona,  showing  no  spots  over  large 
areas.    This  contained  only  0.016  per  cent,  silver. 

These  results  show  that  silver  may  exist  in  galena  to  nearly 
o.  10  per  cent,  in  the  form  of  submicroscopic  particles  or  solid 
solutions.  Isomorphous  mixtures  with  the  silver  minerals  are 
certainly  not  formed.  Above  the  limit  mentioned,  spots  of  silver 
minerals  begin  to  appear  in  constantly  increasing  numbers  as  the 
percentage  of  silver  rises. 

The  method  of  procedure  in  making  the  determinations  de- 
scribed above  consisted  in  selecting  from  the  polished  surfaces 
as  pure  areas  as  possible,  etching  with  nitric  acid,  and  after  ex- 
amin-ng  with  the  reflecting  microscope,  breaking  up  with  a  sharp 
awl  sufficient  of  the  material  for  scorification  and  cupellation. 
From  100  to  500  milligrams  wxre  found  sufficient  except  in  case 
of  the  vei*y  lean  specimens  w^hen  one  gram  was  taken  where  pos- 
sible. The  spots  were  found  to  be  grouped  very  unevenly  so  care 
had  to  be  exercised  in  not  cutting  too  deeply  thus  decreasing  the 
possibility  of  the  sample  representing  different  conditions  from 
those  recorded  on  the  surface. 

Tetrahedrite. — Tetrahedrite  when  argentiferous  (freibergite) 
becomes  an  important  mineral  of  silver.  Analyses  show  as  high 
as  36.90  per  cent,  silver.^^  This  is  present  as  an  isomorphous 
mixture  of  some  silver  molecule.  While  frequently  important 
economically,  it  is  doubly  interesting  from  the  standpoint  of 
paragenesis.  Microscopic  investigation  shows  it  to  be  a  more 
prolific  source  of  the  later  silver  minerals  than  is  galena.  Indeed, 
much  of  the  galena  in  the  silver  deposits  has  replaced  tetra- 
'hedrite  and  its  silver  content  is  thus  due  to  residual  spots  of 
silver-bearing  tetrahedrite  as  well  as  argentite  from  the  break- 
down of  the  complex  molecule. 

Tetrahedrite  has  frequently  been  mentioned  in  geological  de- 

ifs  Hintze,  "  Handbuch  der  Mineralogie,"  Bd.  I.,  p.  11 18. 


'MICROSCOPIC  STUDY  OF  SILVER  ORES.  307 

scriptions  as  an  early  mineral.  In  the  Cripple  Creek  district  it 
persists  to  a  depth  of  2,000  feet  below  the  surface.^"  It  is  an 
early  mineral  at  Butte,  Montana,  although  the  corresponding 
arsenic  compound,  tennantite,  is  thought  to  be  later. ^^  It  is  de- 
scribed as  secondary,  on  the  other  hand,  at  Bingham,  Utah,^^ 
and  Rio  Tinto,  Spain.-*^  It  is  still  more  emphatically  described 
as  a  late  mineral  in  the  Georgetown  Quadrangle,  Colorado.  It  is 
stated  to  be  "the  last  of  the  silver  minerals  to  form."^^  The 
writer  has  invariably  found  tetrahedrite  earlier  than  the  rich 
silver  minerals  and  even  earlier  than  galena,  as  illustrated  in 
Plate  X.,  D.  Its  most  common  relation  to  sphalerite  is  shown  in 
Plate  X.,  C,  2L  photomicrograph  of  material  from  the  Silver  King 
mine  in  Arizona.  Occasionally  it  is  found  actively  replacing 
sphalerite  as  appears  in  Plate  XL,  D,  from  a  specimen  of  the  Sil- 
versmith mine,  Sandon,  B.  C.  Residual  masses  of  sphalerite  are 
frequently  found  so  that  tetrahedrite  may  definiteley  be  placed 
later  than  sphalerite. 

As  suggested  above  tetrahedrite  easily  gives  way  to  attacking 
solutions,  and  it  may  thus  form  the  starting  point  for  a  series  of 
reactions  by  which  minerals  richer  than  itself  in  silver  are  de- 
veloped even  finally  ending  up  with  native  silver.  Thus  in  Plate 
XII.,  A,  also  from  the  Silver  King  mine,  it  is  found  with  a  com- 
plex system  of  veinlets  of  stromeyerite  which  in  places  enlarge 
to  areas  of  considerable  size.  This  type  of  alteration  has  been 
observed  in  many  specimens,  another  example  being  given  in 
XII.,  B,  from  an  unknown  locality  in  Colorado.  The  alteration 
has  set  in  around  quartz  grains  and  along  cracks.  The  further 
breakdown  of  stromeyerite  will  be  described  later.  In  the  break- 
down of  the  tetrahedrite  molecule  illustrated  in  Plate  XII.,  A 

1'^  Lindgren  and  Ransome,  "  Geology  and  Ore  Deposits  of  the  Cripple  Creek 
District,"  Prof.  Paper,  U.  S.  Geol.  Surv.,  No.  54,  p.  121,  1906. 

18  Weed,  Emmons  and  Tower,  U.  S.  Geol.  Surv.,  Butte  Special  Folio,  No. 
38,  p.  6,  1897 

19  Boutwell,  "  Economic  Geology  of  the  Bingham  Mining  District,"  Prof. 
Paper,  U.  S.  Geol.  Surv.,  No.  38,  pp.  107,  220,  1905. 

20  Finlayson,  "  The  Pyrite  Deposits  of  Huelva,  Spain,"  Econ.  Geol.,  Vol. 

5,  P-  411- 

21  Spurr,  Garrey  and  Ball,  "  Economic  Geology  of  the  Georgetown  Quad- 
rangle, Colo.,"  Prof.  Paper  U.  S.  Geol.  Surv.,  No.  63,  p.  261,  1908. 


3oS 


F.  N.  GUILD. 


and  5,  the  sulphide  of  antimony  has  been  removed  leaving  the 
sulphides  of  copper  and  silver,  some  of  which  appear  as  the 
double  salt  Cu,S-Ag,S.  If  the  silver-copper  portion  of  the  tetra- 
hednte  molecule  be  taken  as  3R,S,  Sb.S^r^  and  the  ratio  of 
silver  to  copper  be  considered  as  2:  i,  a  possible  reaction  is  sug- 
gested m  the  equation  below  : 

(Cu^S),,  Ag^S,  Sh,S,  =  Ag,S-Cu,S^Cu,S  +  Sh,S,. 
Later  it  will  be  shown  that  much  of  the  stromeyerite  is  mingled 
with  an  excess  of  the  chalcocite  molecule  so  the  presence  of  the 
extra  copper  sulphide  in  the  above  reaction  is  accounted  for 
What  becomes  of  the  antimony  sulphide  cannot  be  definitely 
stated,  though  it  seems  probable  that  it  enters  into  the  formation 
of  complex  silver  molecules  as  ruby  silver,  polybasite  or  stephan- 
ite  Ihese  minerals  are  often  found  associated  with  tetrahedrite 
and  are  even  found  replacing  it  in  veinlets. 

Tetrahedrite  as  observed  on  polished  surfaces  varies  consid- 
erably m  hardness  and  color,  a  condition  which  causes  some  con- 
fusion m  identification.     This  is  due  to  its  great  variability  in 
composition.    As  shown  by  tables  of  analyses  in  Dana's  "  System 
of  Mineralogy,"  copper  varies  from  10.8  per  cent,  to  4408  per 
cent.;  silver  from  a  trace  to  31.29  per  cent.;  zinc  from  nothing 
to  7.25  per  cent.;  and  iron  from  0.64  per  cent,  to  8.24  per'cent 
Polytehte  is  thought  to  belong  here,  being  a  lead-zinc  variety  with 
less  than  i  per  cent,  copper.    Micro-chemical  tests  on  small  frag- 
ments secured   from  polished  surfaces  show  that  much  of  the 
silver-bearing  variety  is  low  in  copper.    In  addition  to  the  causes 
m  variability  mentioned  above  the  mineral  grades  into  tennan- 
tite,  the  arsenical  variety  of  fahlerz.    Although  it  varies  in  hard- 
ness from  3  to  4,  all  varieties  show  more  relief  than  the  silver 
minerals,  a  very  important  feature  in  identification. 

Separate  determinations  of  silver  have  been  made  on  tetra- 
hedrite and  galena  grains  in  specimens  of  typical  rich  silver  ores 
from  50  to  100  milligrams  of  the  material  being  secured  from  the 

^2  Prior  and  Spencer.  "  Chemical  Composition  of  Fahlerz,"  Min.  Mag    Vol 
I-,  P-  193-     Ihese  authors  propose  the  formula, 

3R'.S,  Sb,(As,)S3-hx(6R",S,  Sb,(As,)S3). 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  309 

polished  surfaces  by  means  of  a  sharp  awl.  The  tetrahedrite 
even  under  high  power  was  seen  to  be  perfectly  homogeneous 
showing  the  silver  to  be  isomorphously  mixed,  while  the  galena 
as  usual  showed  spots.  In  a  specimen  from  the  Nettie  L.  mine, 
Ferguson,  B.  C,  tetrahedrite  grains  were  found  to  contain  7.27 
per  cent,  silver  while  the  galena  contained  0.23  per  cent.  A  sim- 
ilar specimen  from  the  Silver  King  mine,  Arizona,  showed  6.06 
per  cent,  silver  for  the  tetrahedrite  grains,  while  the  galena  con- 
tained 1.24  per  cent.  These  grains  of  galena,  however,  contained 
spots  of  tetrahedrite  that  could  be  seen  by  a  hand  lens.  Still  an- 
other specimen  of  tetrahedrite  from  the  Reco  mine,  Sandon, 
B.  C,  showing  on  polished  surfaces  many  spots  and  veinlets  of 
ruby  silver  (proustite),  contained  25.02  per  cent,  silver.  These 
grains  had,  of  course,  been  enriched  by  late  additions. 

IV.    THE  LATE  SILVER  MINERALS. 

Stromeyerite,  Cu2S-Ag2S. — Stromeyerite  is  one  of  the  rarer 
ore  minerals  of  silver  and  but  little  reference  regarding  its  char- 
acteristics or  paragenesis  is  found  in  published  descriptions  in 
which  the  reflecting  microscope  is  employed.  Murdoch  describes 
it  as  having  a  purplish  tint  when  viewed  side  by  side  with  chalco- 
cite  and  to  have  sometimes  a  smooth  and  sometimes  a  ragged 
surface.  He  also  reports  that  it  is  etched  by  nitric  acid  and 
develops  cleavage  in  some  grains. ^^ 

In  the  silver  ores  thus  far  studied  stromeyerite  has  been  found 
to  present  two  types  of  occurrences.  First,  in  veinlets  and  areas 
of  considerable  size  associated  with  tetrahedrite  (freibergite)  and 
doubtless  derived  from  it.  This  type  has  already  been  described 
under  tetrahedrite  and  is  illustrated  in  Plate  XII.,  A  and  B.  Sec- 
ond, as  a  replacement  product  of  bornite.  In  this  type  it  seems 
to  play  the  same  role  as  chalcocite  in  copper  ores.  There  are  scat- 
tered residual  grains  of  pyrite  almost  completely  replaced  by 
chalcopyrite,  which  in  its  turn  has  altered  to  bornite,  later  to  be 
replaced  by  stromeyerite  and  chalcocite.  All  of  these  trans- 
formations with  the  exception  of  the  first  is  illustrated  in  Plate 

23  Murdoch,  "  The  Microscopic  Determination  of  the  Opaque  Minerals,"  p. 
117,  1916. 


3IO  F.  N.  GUILD. 

XII.,  Cj  a  photomicrograph  of  a  polished  specimen  from  the  Silver 
King  mine,  Arizona.  Pyrite,  although  present  in  the  specimen, 
does  not  appear  in  the  photograph.  An  incipient  replacement  of 
bornite  in  a  specimen  from  the  same  locality  is  illustrated  in  Plate 
XII.,  D.  This  presents  the  appearance  of  a  crude  graphic  struc- 
ture. Replacements  less  often  observed  are  galena  by  stromeyer- 
ite,  when  it  takes  place  along  borders,  and  chalcopyrite  by  stro- 
meyerite.  An  interesting  case  of  the  latter  replacement  was 
observed  in  a  specimen  from  Mt.  Lyell,  Tasmania,  and  is  illus- 
trated in  Plate  XIII.,  A.  The  replacement  has  progressed  in  such 
a  manner  as  to  give  rise  to  the  so-called  graphic  intergrowth  so 
frequently  described.  A  similar  "  intergrowth  "  between  chalco- 
cite  and  bornite  has  been  described  from  this  locality  by  Gilbert 
and  Pogue.^^  These  authors  held  that  the  structure  indicated 
contemporaneous  deposition  of  the  two  sulphides.  These  struc- 
tures are  beautifully  developed  in  several  varieties  of  the  silver 
minerals  and  will  be  discussed  more  fully  later. 

The  polished  surfaces  of  stromeyerite  in  many  of  the  specimens 
studied  show  a  peculiar  complicated  structure  which  has  been  the 
subject  of  considerable  investigation  to  determine  its  origin.  It 
occurs  most  frequently  as  an  intricate  mass  of  blades  somewhat 
resembling  oleander  leaves  in  shape,  arranged  in  great  confusion 
or  roughly  grouped  in  designs.  The  blades  have  a  perceptible 
purplish  tint  and  are  slightly  softer  than  the  groundmass.  The 
structure  is  best  brought  out  by  polishing  on  the  felt  wheel  so  as 
to  develop  considerable  relief.  By  the  use  of  a  red  screen  to 
darken  the  purplish  tint,  the  structure  may  be  brought  out  fairly 
well  for  photographic  purposes  as  shown  in  Plate  XII.,  C.  On 
etching  with  potassium  cyanide  solution,  however,  the  design  is 
brought  out  in  a  striking  manner  as  appears  in  Plate  XIII.,  B.  The 
harder  and  lighter  tinted  material  constituting  the  background  of 
the  design  is  more  severely  acted  upon  by  the  reagent  and  appears 
nearly  black.  The  blades  are  only  slightly  affected,  so  that  by 
rubbing  lightly  on  cloth  they  become  quite  free  from  the  black 
coating  that  first  appears.    The  structure  as  shown  in  Plate  XIII., 

24  Gilbert  and  Pogue,  "  Mt.  Lyell  Copper  District,"  Proc.  Nat.  Mus.,  Vol. 
45,  P-  609. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  311 

B,  is  often  very  uniformly  developed  throughout  a  considerable 
area,  although  occasionally  both  the  purplish  blades  and  the 
whiter  groundmass  enlarge  to 'patchy  masses.  This  is  illustrated 
in  Plate  XIII.,  C  and  D,  photomicrographs  from  a  specimen  from 
an  unknown  locality  in  Arizona.  Each  area  is  smooth,  the 
transition  sometimes  being  through  the  bladed  structure  while 
at  others  more  abrupt.  On  etching  with  potassium  cyanide 
solution  the  soft  smooth  areas  are  only  slightly  acted  upon 
while  the  faintly  harder  portion  marked  "  cc "  is  blackened  and 
cleavage  lines  are  developed  (Plate  XIII.,  C).  The  transition 
borders  etch  exactly  as  is  found  to  be  the  case  in  many  of  the 
other  specimens  of  stromeyerite  (Plate  XIII.,  5).  The  ex- 
planation of  this  structure  is  that  the  design  is  caused  by  a  mixture 
of  stromeyerite  and  chalcocite,  the  former  constituting  the  purplish 
blades  which  are  thus  set  in  a  background  of  chalcocite.  This 
opinion  is  practically  substantiated  by  both  qualitative  and  Cjuan- 
titative  tests.  The  purplish  surface  marked  ''  st"  reacts  strongly 
for  silver  in  addition  to  copper,  while  the  area  marked  "  cc " 
(XIII.,  C),  reacts  for  silver  but  slightly.  Two  specimens  of 
stromeyerite  have  been  found  which  do  not  show  the  complicated 
structure,  one  being  from  Tombstone,  Arizona,  and  the  other 
from  Mt.  Lyell,  Tasmania.  The  material  from  Tombstone  was 
not  adapted  for  quantitative  determinations,  as  it  was  found 
under  high  power  to  be  filled  with  minute  specks  of  native  silver, 
probably  a  breakdown  product  of  the  silver  mineral.  Fairly  pure 
material  could  be  secured  from  the  polished  surface  of  the  Tas- 
mania specimen,  which  yielded,  after  scorification  and  cupellation, 
44.40  per  cent,  silver.  The  amount  of  material  worked  upon  was 
only  13.8  milligrams.  It  still  contained  a  few  admixed  particles 
of  chalcopyrite  which  could  not  be  removed.  Material  secured 
in  the  same  manner  from  surfaces  showing  the  complicated 
structure  contained  only  23.48  per  cent,  silver,  which  corresponds 
to  44.2  per  cent,  of  the  double  molecule,  AgoS,  CugS,  the  re- 
mainder doubtless  being  chalcocite.  The  low  percentage  of 
silver  in  the  material  showing  a  homogeneous  surface  (theory 
requires  53.05  per  cent.)  may  be  due  to  the  chalcopyrite  or  some 
chalcocite   in   solid   solution.      These   results  together  with   the 


312  F.  N.  GUILD. 

microscopic  evidence  are  considered  conclusive  ground  for  re- 
garding the  complicated  structure  observed  in  some  specimens  of 
stromeyerite  to  be  due  to  a  complex  mixture  of  the  chalcocite 
molecule  and  stromeyerite. 

The  two  components  making  up  the  stromeyerite  structure 
having  been  identified,  it  now  remains  to  discuss  the  causes  which 
have  developed  the  interesting  feature.  In  this  matter  little  more 
can  be  done  than  to  suggest  the  possibilities.  First,  it  may  repre- 
sent the  unmixing  of  a  solid  solution,  or  breakdown  of  an  iso- 
morphous  mixture.  According  to  this  theory  the  two  sulphides 
AgoS  and  CuoS  were,  under  the  conditions  of  formation,  first 
deposited  as  an  isomorphous  mixture  or  solid  solution.  Later  as 
conditions  changed  the  material  found  itself  unstable  and  broke 
up  into  the  two  molecules  CugS-AggS  and  CuoS.  The  results  of 
microscopic  examinations  show  that  under  present  conditions 
silver  sulphide  and  copper  sulphide  do  not  remain  mixed  in  all 
proportions.  In  other  words  stromeyerite  appears  to  be  a  definite 
double  salt.  Transition  between  it  and  chalcocite  is  not  charac- 
terized by  a  fading  ofif  of  one  into  the  other  but  by  the  complex 
mingling  of  distinct  individuals  (Plate XIII.,  C  and  D).  Further, 
published  chemical  analyses  seem  to  show  the  ratio  of  copper 
sulphide  to  silver  sulphide  to  remain  rather  persistently  near  the 
ratio  I  :  I.  Thus  in  the  table  of  analyses  given  by  Hintze,  eight 
out  of  seventeen  correspond  very  closely  to  the  ratio  given.  Seven 
have  an  excess  of  copper  and  two  an  excess  of  silver.^^  Were  the 
mineral  an  isomorphous  mixture  there  would  certainly  have  been 
greater  variation.  Jalpaite,  a  mineral  of  the  formula  3Ag2S, 
CugS,  and  isometric  in  crystallization,  has  been  described  from 
Jalpa,  Mexico,  and  Tres  Puntas,  Chile. ^'^  Margottet  obtained  in 
the  laboratory  isometric  crystals  corresponding  to  the  formulae 
2Ag2S,  CuoS  and  3Ag2S,  CuoS,  the  latter  having  the  composition 
of  jalpaite."^  These  facts  suggest  the  possibility  of  various 
combinations  taking  place  under  proper  conditions  other  than 

25  Hintze,  "Handbuch  der  Mineralogie,"  Bd.  i,  p.  542. 

26  Ibid.,  Bd.  I,  p.  458. 

27  Quoted  by  Hintze,  "Handbuch  der  Mineralogie,"  Bd.  i,  p.  542. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  3^3 

that  expressed  by  the  ratio  i :  i.     These  on  breaking  down  might 
develop  a  compHcated  structure. 

Second,  the  structure  may  be  caused  by  replacement,  a  theory 
strongly  suggested  in  Plate  XIII.,  C  and  D.  Then  the  complex 
borders  are  replacement  phenomena  resulting  from  the  attack  of 
silver  solutions  upon  chalcocite  or  copper  solutions  upon  stro- 
meyerite.  The  occurrence  of  stromeyerite  around  the  borders  of 
chalcocite  areas,  as  is  sometimes  observed,  also  favors  this  theory. 

Third,  the  structure  may  have  resulted  from  the  segregation 
or  recrystallization  of  metacolloidal  material.  According  to  this 
theory  the  sulphides  of  silver  and  copper  resulting  from  the 
breakdown  of  freibergite  or  other  minerals  are  at  first  mingled 
in  a  colloidal  condition  which  on  becoming  crystalline  develop 
the  peculiar  structure  described.  This  is  the  theory  rather  favored 
by  the  writer,  thoug-h  it  is  thought  that  any  one  of  the  three  proc- 
esses may  have  been  active  in  individual  cases.  Then  the  struc- 
ture simply  represents  a  final  state  of  equilibrium  reached  when- 
ever these  materials  are  brought  together  by  whatever  causes. 
Even  if  the  first  mentioned  theory  be  the  correct  one,  segregation 
would  have  to  take  place  before  the  constituents  of  the  break- 
down could  develop  the  present  structure. 

A  beautiful  so-called  intergrowth  between  stromeyerite  and 
galena  of  quite  a  different  design  has  been  observed  in  specimens 
from  the  Silver  King  mine,  Arizona  (Plate  XIV.,  A),  and  from 
Cobalt,  Ontario.  In  appearance  it  is  much  like  the  eutectic  struc- 
ture of  alloys.  It  is  very  fine  and  requires  an  oil-immersion  lens 
to  bring  out  the  structure  satisfactorily.  In  only  one  specimen 
had  the  white  areas  developed  sufficiently  to  show  the  charac- 
teristic triangular  pits  of  galena.  The  small  areas  showing  the 
pseudo-eutectic  structure  are  sometimes  angular,  as  though 
originally  a  definite  crystal,  while  at  others  the  borders  are  ex- 
tremely irregular. 

In  the  attempt  to  explain  the  origin  of  this  structure  the  same 
three  possibilities  mentioned  above  might  be  considered.  Since  it 
is  a  deposition  from  solution  it  cannot  in  any  way  be  looked  upon 
as  a  true  eutectic. 

Graphic  intergrowths  have   frequently  been  described  in  the 


SH  F.  N.  GUILD. 

journals,  and  until  recently  have  been  held  by  most  authors  to  be 
due  to  simultaneous  deposition  of  the  two  sulphides.  This  has 
been  in  accordance  with  Laney's  hypothesis  advanced  in  1911.^^ 
Rogers  in  1914  suggested  that  the  structure  was  due  to  replace- 
ment-^ and  in  19 16  described  material  from  several  new  localities 
and  showed  conclusively  that  in  the  cases  cited  by  him,  his 
explanation  was  correct. ^°  Whitehead  had  also  arrived  at  the 
same  conclusion. ^^  The  intergrowths  of  galena  and  stromeyerite 
here  described  are  much  more  intricate  than  those  thus  far  ex- 
plained and  the  evidence  that  they  are  due  to  replacement  is  not 
at  all  conclusive.  No  transition  types  have  been  observed  by 
which  a  clue  to  the  origin  of  the  structure  could  be  ascertained. 
The  writer  is  inclined  to  think  that  it  is  due  to  a  readjustment  of 
subcrystalline  or  colloidal  mixture  which  may  have  resulted  from 
a  breakdown  of  solid  solution  or  been  an  original  precipitate  from 
aqueous  solution. 

Stromeyerite  with  an  excess  of  the  argentite  molecule  ap- 
parently develops  a  structure  that  can  hardly  be  distinguished 
from  the  stromeyerite-galena  intergrowth.  This  has  been  ob- 
served in  a  specimen  from  Mt.  Lyell,  Tasmania. 

Pyrargyrite,  sAg2S,  vS&2-S"3  and  Proustite,  3Ag2S,  As^S^^. — The 
ruby  silvers  rank  among  the  most  important  silver  minerals  in 
many  of  the  rich  deposits  of  the  United  States  and  elsewhere. 
Invariably  these  minerals  are  thought  to  have  been  deposited 
late  in  the  history  of  ore  deposition.  Thus  Spurr  describes 
pyrargyrite  as  coating  crevices  that  cut  the  primary  ore.^-  It  is 
also  found  in  the  oxidized  material  and  is  thought  to  have  been 
deposited  by  descending  solutions.  In  the  same  district  argen- 
tite, polybasite  and  stephanite  are  thought  in  part  to  be  "primary" 

28  Laney,  "  The  Relation  of  Bornite  and  Chalcocite  in  the  Copper  Ores  of 
the  VirgiHna  District  of  North  Carolina  and  Virginia,"  Econ.  Geol.,  Vol.  6, 
p.  399,  191 1. 

23  Rogers,  Mitt,  and  Scientific  Press,  Vol.  108.  p.  609,  1914- 

30  Rogers,  "The  So-called  Graphic  Intergrowth  of  Bornite  and  Chalcocite," 
Econ.  Geol.,  Vol.  11,  p.  582,  1916. 

31  Whitehead,  "  The  Paragenesis  of  Certain  Sulphide  Intergrowths,"  Econ. 
Geol.,  Vol.  11,  p.  i,  1916. 

^-  Spurr,  "  Geology  of  the  Tonopah  Alining  District,"  U.  S.  Geol.  Surv. 
Prof.  Paper,  No.  42,  1905. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  315 

minerals.  Irving  and  Bancroft  find  it  associated  with  the  latest 
deposited  minerals  of  the  vein  and  restricted  to  the  upper  levels 
of  the  mine.^^  Ransome  holds  that  the  ru'by  silver  minerals  to- 
gether with  stephanite  and  polybasite  are  as  characteristic  of 
downward  enrichment  as  chalcocite.^^  As  already  pointed  out 
the  early  minerals  of  silver  are  confined  mainly  if  not  entirely  to 
tetrahedrite  and  argentiferous  galena.  They  are  therefore  held 
to  be  the  source  of  the  later  enriched  products.  Tetrahedrite  is 
probably  the  most  prolific  source  as  shown  by  microscopic  inves- 
tigations, and  the  fact  that  the  late  silver  minerals  are  most  often 
arsenic  or  antimony  compounds.  Thus  the  results  obtained 
from  the  study  of  polished  surfaces  are  in  accord  with  the  earlier 
geological  observations,  at  least,  in  respect  to  the  late  deposition 
of  these  minerals.  They  are  observed  replacing  sphalerite,  tetra- 
hedrite and  galena. 

Proustite  replacing  galena  which  in  its  turn  has  replaced  pyrite 
and  arsenopyrite  is  illustrated  in  X.,  B,  a  photomicrograph  of  a 
polished  specimen  from  an  unknown  locality  in  Mexico.  Its  oc- 
currence as  spots  in  argentiferous  galena  (Plate  XL,  B)  has 
already  been  described.  A  veinlet  of  pyrargyrite  in  galena  is 
shown  in  Plate  XL,  C.  Proustite  and  galena  occurring  as  veinlets 
in  sphalerite  is  illustrated  in  Plate  XIV.,  B,  the  specimen  being 
from  Schemnitz,  Hungary.  Stephanite  is  found  in  other  portions 
of  the  same  section  (A)  and  exhibits  similar  features.  Photo- 
graphs might  be  added  illustrating  how  the  ruby  silvers  re- 
place tetrahedrite  in  the  same  manner.  It  is  occasionally  ob- 
served breaking  down  into  native  silver  (Plate  XIV.,  C,  Durango, 
Mexico).  In  a  specimen  of  argentite  from  Freiberg,  Saxony,  it 
appears  replacing  the  silver  sulphide  in  spots  and  borders  around 
impurities. 

In  a  specimen  from  Tonopah,  Nev.,  pyrargyrite  and  proustite 
are  associated  with  each  other  in  an  interesting  manner  as  illus- 
trated in  Plate  XIV.,  D,  and  Plate  XV.,  A.    These  are  evidently 

33  Irving  and  Bancroft,  "Geology  of  the  Ore  Deposits  near  Lake  City, 
Colo.,"  Bui.  U.  S.  Geol.  Surv.,  No.  478,  p.  63,  191 1. 

2*  Ransome,  "  Criteria  of  Downward  Sulphide  Enrichment,"  Econ.  Geol., 
Vol.  5,  p.  211,  1910. 


3l6  F.  N.  GUILD. 

cavity  fillings,  as  shown  by  drusy  quartz  borders,  frequently  with 
sharp  euhedral  crystals.  Empty  cavities  are  also  present  in  the 
specimen,  and  the  arrangement  of  the  quartz  grains,  as  shown  in 
thin  section,  also  favors  this  explanation.  In  Plate  XIV.,  D,  pyr- 
argyrite  occupies  the  central  portion,  the  borders  being  proustite, 
the  earlier  of  the  two  to  be  deposited.  Proustite  appears  faintly 
bluer  than  pyrargyrite  in  reflected  light,  yet  when  freshly  polished 
the  contrast  is  so  slight  as  to  be  just  barely  perceptible.  It  would 
have  been  impossible  to  represent  this  occurrence  photographically 
were  it  not  for  the  peculiar  differential  action  of  the  electric  arc. 
The  central  portion,  pyrargyrite,  is  rapidly  tarnished  by  the  un- 
protected arc,  bright  effects  being  first  produced  along  the  border, 
continued  action  causing  a  blacking  of  the  whole  surface.  When 
the  arc  is  of  low  intensity  or  protected  by  ground  glass  or  the 
daylight  screen  employed  for  visual  work,  little  or  no  tarnishing 
effect  is  observed.  The  blackened  surface  was  examined  under 
an  oil-immersion  lens,  when  a  perceptible  rectangular  design 
could  be  made  out.  This  feature  has  been  observed  in  other  silver 
minerals,  notably  in  some  samples  of  argentite.  The  blackening 
develops  at  times  with  such  rapidity  as  to  make  photographing 
difficult.  In  Plate  XV.,  A,  the  arrangement  is  reversed,  proustite 
occupying  the  center  with  an  incomplete  border  of  pyrargyrite. 
In  the  lower  portion  of  the  photograph  tetrahedrite,  the  mineral 
showing  relief,  appears  as  a  thin  rim  between  the  two.  This  en- 
larges and  forms  the  whole  of  the  border  in  the  upper  portion.  It 
is  thought  that  the  cavity  was  first  filled  with  tetrahedrite  which 
was  later  nearly  replaced  by  proustite,  leaving  a  narrow  border. 
Later  pyrarg}a-ite  came  in,  replacing  tetrahedrite  along  the  mar- 
gin of  the  projecting  cjuartz  crystals,  still  leaving  a  residual  rim 
of  tetrahedrite.  In  the  upper  portion  of  the  photograph,  pyrar- 
gyrite failed  to  appear.  If  this  explanation  is  correct,  the  order 
of  deposition  of  the  two  ruby  silvers  is  the  same  in  each  case  and 
they  are  both  later  than  tetrahedrite  as  usual. 

These  minerals  were  identified  by  micro-chemical  tests,  as  well 
as  by  simple  microscopic  tests  of  small  fragments  broken  up  from 
the  polished  surface  by  a  sharp  point.  Proustite  is  light  red  in 
thin  fragments,  while  pyrargyrite  is  deep  cherry  red  on  the  thin 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  317 

borders,  approaching  opacity  in  the  thick  centers.  The  streaks 
are  also  very  characteristic,  proustite  being  cherry  red  and  pyrar- 
gyrite  dark  purpHsh  red  or  brown.  This  test  may  also  be  made 
by  crushing  small  fragments  on  glazed  paper.  Tetrahedrite  re- 
acts for  copper  and  shows  higher  relief  than  any  of  the  silver 
minerals.  '         ^^  -.  -  ,  -■-■■  ;- 

Other  cavity  fillings  similar  to  those  described  above,  in  which 
the  two  ruby  silvers  were  mingled  in  a  very  intricate  manner, 
were  observed  in  specimens  from  the  same  locality.  A  striking 
feature  was  the  sharp  boundary  between  the  two  minerals,  a 
grading  ofif  of  one  into  the  other  never  having  been  observed. 
This  is  in  accordance  with  the  investigations  of  Miers,  who  from 
crystallographic  and  chemical  considerations,  concluded  that 
proustite  and  pyrargyrite  formed  distinct  species  and  are  not 
perfectly  isomorphous.^^ 

In  a  specimen  containing  the  ruby  silvers  from  Schemnitz, 
Hungary,  proustite  was  found  in  beautiful  so-called  intergrowth 
with  galena  (Plate  XV.,  B).  The  small  area  showing  the  pseudo- 
eutectic  structure  was  completely  enclosed  by  pyrargyrite,  which 
on  exposure  to  the  electric  arc  became  darkened,  as  appears  in 
the  illustration.  The  elongated  area  showing  the  unusual  struc- 
ture seems  to  be  a  residual  mass  remaining  after  replacement  by 
pyrargyrite.  The  galena  portions  project  slightly  into  the  pyrar- 
gyrite, showing  an  inequality  in  the  ease  of  replacement.  The 
origin  of  structures  of  this  kind  where  transitionary  stages  have 
not  been  found  is  still  in  doubt. 

Plate  XV.,  C,  was  taken  from  a  veinlet  consisting  of  an  intricate 
mixture  of  pyrargyrite  and  proustite  in  calcite  from  the  Cobalt 
district  of  Canada.  Marcasite  has  developed  in  fine  divergent 
forms  resembling  organic  growths.  It  is  believed  that  the  iron 
disulphide  has  developed  later  than  the  silver  minerals,  a  very 
rare  occurrence  for  this  substance  as  discussed  more  fully  under 
pyrite.  At  Goldfield,  Nev.,  however,  it  frequently  appears  later 
than  the  other  sulphides  of  the  district. ^^ 

35  Miers,  "  Contribution  to  the  Study  of  Pyrargyrite  and  Proustite,"  Min. 
Mag.,  Vol.  8,  p.  57. 

36  Ransome,   "  The   Geology  and   Ore  Deposits   of  Goldfield,   Nev.,"   Prof. 
Paper,  U.  S.  Geol.  Surv.,  No.  66,  p.  115,  1909. 


318  F.  N.  GUILD. 

Stephanite,  ^Ag^S,  ShoS-^. — Stephanite  is  one  of  the  less  com- 
mon minerals  of  silver  and  often  when  fomid  in  association  with 
other  sulphides  is  in  such  small  particles  as  not  to  permit  a  satis- 
factory study  of  its  paragenesis.  It  is  a  late  mineral  and  in  gen- 
eral is  found  in  occurrences  similar  to  the  ruby  silvers  and  other 
sulpho-salt  minerals.  The  factors  which  have  caused  this  mineral 
to  develop  rather  than  pyrargyrite  have  not  yet  been  worked  out. 
It  may  be  due  merely  to  the  relative  concentration  of  silver  and 
antimony  in  the  vein  solutions.  Stephanite  with  kernels  of  pyr- 
argyrite have  been  described  from  Freiberg,  Germany,^'^  and 
many  other  cases  are  on  record  where  it  is  found  in  intimate  asso- 
ciation with  the  sulpho-salt  minerals  of  silver.  It  seems  to  show 
rather  a  strong  tendency  to  replace  gangue  material.  Thus  Fen- 
ner  has  described  an  interesting  occurrence  of  this  mineral  from 
Colorado,  in  which  the  constituents  of  rhyolite  porphyry  have 
been  replaced  by  stephanite  and  chalcopyrite.  In  one  section  de- 
scribed chalcopyrite  replaced  the  groundmass,  while  stephanite 
appeared  later  replacing  feldspar  phenocrysts.^^  Bastin  has  men- 
tioned stephanite  as  a  rare  constituent  associated  with  polybasite, 
proustite,  galena,  etc.,  in  the  later  minerals  of  the  silver  veins  of 
Gilpin  Co.,  Colo.^^ 

Stephanite  replacing  fragments  in  brecciated  material  was  ob- 
served in  a  specimen  from  the  Reco  mine,  Sandon,  B.  C,  and  is 
illustrated  in  D.  The  brecciated  material  consisted  of  quartz 
with  calcite  cement  and  formed  an  interesting  mosaic  with  pyrite, 
sphalerite,  galena  and  stephanite.  The  ore  minerals  were  too 
disseminated  in  this  specimen  to  permit  a  determination  of  the 
order  of  deposition.  In  another  section  from  the  same  locality 
(Plate  X.,  A)  it  is  seen  in  close  association  with  other  sulphides. 
Pyrite  is  fractured  and  replaced  to  small  degree  by  chalcopyrite 
in  the  form  of  borders  and  veinlets.  Sphalerite  is  later  than 
pyrite  and  is  replaced  by  galena.  Stephanite  is  then  deposited 
after  which  a  later  generation  of  chalcopyrite  crosses  both  galena 

37  Hintze,  "  Handbuch  der  Mineralogie,"*Bd.  I.,  p.  1155. 

38  Fenner,  "A  Replacement  of  Rhyolite  Porphyry  by  Stephanite  and  Chal- 
copyrite at  Leadville,"  Sc.  Mines  Quar.,  Vol.  31,  p.  235,  1909-10. 

39  Bastin,    "  Metasomatism    in    Downward    Sulphide    Enrichment,"    EcoN. 
Geol.,  Vol.  8,  p.  51,  1913. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  3^9 

and  stephanite.  The  order  of  deposition  then  is:  (i)  Pyrite, 
(2)  sphalerite,  (3)  chalcopyrite,  (4)  galena,  (5)  stephanite,  and 
(6)  late  chalcopyrite.  Still  another  specimen  showing  sphalerite, 
galena  and  stephanite,  also  from  the  same  district,  is  illustrated  in 
Plate  XVI.,  A. 

Stephanite  may  be  identified  on  polished  surfaces  by  its  in- 
ferior hardness,  gray  color,  duller  than  galena,  black  fragments 
absolutely  opaque  when  examined  under  a  high-power  microscope 
with  transmitted  light,  micro-chemical  reaction  for  silver  and 
failure  to  respond  to  the  test  for  copper  (tetrahedrite  and 
polybasite). 

Polybasite,  pAg^S  {CU2S)  Sh^S^. — Polybasite  does  not  show 
any  paragenetic  features  that  differentiate  it  from  the  other  anti- 
mony or  arsenic  compounds  of  silver.  It  is  later  than  galena  and 
appears  cutting  the  still  older  sulphides,  sphalerite,  and  tetrahe- 
drite. At  Tonopah,  Nev.,  according  to  Spurr,  it  is  found  as  deep 
as  500  feet  below  the  surface  and  may  be  deposited  by  ascending 
solutions.'*'^  It  is  described  by  Bastin  as  a  secondary  mineral  re- 
placing galena.^^  The  corresponding  arsenic  mineral,  pearceite, 
has  been  observed  by  Whitehead,  in  specimens  from  the  Gemini 
mine,  Tintic,  Utah,  in  graphic  intergrowth  with  galena.^^  It  is 
believed  to  be  replacing  galena.  In  the  Little  Belt  mountains, 
Mont.,  it  has  been  described  as  "an  alteration  product  of  galena, 
and  to  be  mixed  with  and  to  grade  into  pyrargyrite,  which  in 
some  cases  is  its  undoubted  alteration  product."'*^ 

Several  occurrences  similar  to  those  described  above  have  been 
studied  by  the  writer.  Thus  it  is  illustrated,  associated  with 
galena  in  a  veinlet  crossing  sphalerite,  in  Plate  XVL,  B,  a  photo- 
micrograph of  a  polished  specimen  from  Guanacevi,  Mexico. 
Here  it  has  replaced  galena,  which  has  earlier  replaced  sphalerite. 

^0  Spurr,  "  Geology  of  the  Tonopah  Mining  District,"  Prof.   Paper,  U.  S. 
Geol.  Surv.,  No.  42,  p.  95,  1905. 

41  Bastin,    "  Metasomatism    in    Downward    Sulphide    Enrichment,"    EcoN. 
Geol.,  Vol.  8,  p.  51,  1913. 

42  Whitehead,  "Paragenesis  of  Certain  Sulphide  Intergrowth s,"  EcoN.  Geol., 
Vol.  II.  p.  I,  1916. 

43  Weed  and  Pirsson,  "  Geology  of  the  Little  Belt  Mountains,  Mont.,"  20th 
Ann.  Kept.  U.  S.  Geol.  Surv.,  Pt.  3,  p.  411,  1899. 


320  F.  N.  GUILD. 

In  a  specimen  from  the  Seven  Thirty  mine  near  Georgetown, 
Colo.,  tetrahedrite  appeared  as  residual  grains  surrounded  by 
polybasite  in  which  were  found  numerous  wavy  streaks  of  proust- 
ite.  The  two  silver  minerals  were  doubtless  alteration  products 
of  arsenical  tetrahedrite. 

Polybasite  as  viewed  on  polished  surfaces  is  considerably  duller 
than  galena  and  may  easily  be  distinguished  from  tetrahedrite, 
which  it  closely  resembles  in  color,  by  its  inferior  relief.  It  is 
distinguished  from  stephanite  and  pyrargyrite  by  the  presence 
of  copper,  which  is  always  in  isomorphous  mixture  with  the 
silver.  It  is  sometimes  cherry  red  on  the  thinnest  edges  like 
pyrargyTite. 

Argentite,  Ag2S. — From  the  numerous  descriptions  of  the  oc- 
currence of  argentite,  it  would  seem  that  it  may  be  either  a 
hypogene  or  supergene  mineral.  Thus  great  masses  were  found 
in  the  upper  zones  of  the  Comstock  lode.  Such  bonanzas  usually 
directly  underlie  the  oxidized  material  and  the  evidence  seems 
to  be  conclusive  that  they  were  deposited  by  supergene  solutions. 
Yet  in  the  Comstock  lode,  for  example,  argentite  has  been  iden- 
tified at  a  depth  of  3,000  feet  below  the  surface. ^^  At  Tonopah, 
Nev.,  also  some  occurrences  are  held  to  be  primary  while  others 
are  thought  to  be  secondary.^^  Many  other  examples  might  be 
cited  showing  that  there  is  abundant  evidence  in  support  of  both 
the  hypogene  and  supergene  origin  of  this  mineral.  In  both  of 
these  cases,  however,  it  was  a  late  mineral  to  be  deposited,  prob- 
ably just  later  than  galena  and  earlier  than  second  generation 
chalcopyrite. 

Emmons  has  expressed  surprise  that  argentite  is  not  more 
often  found  metasomatically  replacing  other  sulphides,  a  reaction 
which  he  believes  should  take  place  on  account  of  its  position  as  the 
lowest  member  in  the  solubility  scale.^^  In  the  specimens  studied 
during  this  investigation,  argentite  is  rarely  found  to  have  actively 
replaced  the  earlier  sulphides.     It  is  most  often  found  in  small 

4*  Emmons,  "  The  Enrichment  of  Sulphide  Ores,"  Bui.  U.  S.  Geol.  Surv., 
No.  529,  p.  120,  1913. 

*^  Spurr,  "  Geology  of  the  Tonopah  Mining  District,  Nevada,"  U.  S.  Geol. 
Surv.  Prof.  Paper,  No.  42,  p.  22,  1905. 

*"  Emmons,  loc.  cit.,  p.  121. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  321 

particles  filling  cavities  in  gangue  material,  and  associated  with 
galena,  and  the  late  silver  minerals  as  pyrargyrite,  proustite,  etc. 
On  the  other  hand,  argentite  seems  to  yield  easily  to  the  attack 
of  vein  solutions.  Almost  every  specimen  shows  the  breakdown 
into  native  silver  to  greater  or  less  degree.  This  may  occur  as 
borders  reaching  out  more  or  less  into  the  cracks  of  adjacent 
minerals,  as  delicate  wandering  veinlets,  or  it  may  be  recrystal- 
lized  in  large  crude  masses.  One  specimen  from  Butte,  Montana, 
showed  thousands  of  minute  specks  of  silver  evenly  sprinkled 
throughout  the  mass,  all  of  such  minute  size  that  they  were  in- 
visible except  under  high  power.  These  occurrences  will  be  more 
fully  described  in  the  section  on  native  silver. 

Replacement  of  argentite  by  a  late  generation  of  pyrite  has 
already  been  mentioned.  This  observation  was  made  on  a  spec- 
imen from  Freiberg,  Saxony,  and  is  illustrated  in  Plate  XVI.,  C. 
A  late  generation  of  chalcopyrite  has  frequently  been  observed 
replacing  the  rich  silver  minerals.  Argentite  was  found  to  be  no 
exception.  Thus  in  Plate  XVI.,  D,  we  have  an  illustration  of 
cavity  filling  by  argentite,  from  Butte,  Mont.,  where  the  borders 
have  become  delicately  replaced  by  a  fringe  of  chalcopyrite  prob- 
ably supergene.  The  argentite  has  purposely  been  darkened  by 
the  action  of  the  electric  arc,  in  order  to  show  contrast  between 
the  argentite  and  chalcopyrite.  The  same  specimen  shows  minute 
dots  and  dashes  of  ruby  silver,  which  is  probably  a  late  replace- 
ment product. 

Argentite  replacing  smaltite  in  a  complex  system  of  veinlets 
has  been  observed  in  specimens  from  Cobalt,  Ont.,  and  is  illus- 
trated in  Plate  XVII.,  A.  Patches  of  stromeyerite  occur  in  the 
argentite  areas.  Both  are  replaced  here  and  there  by  native 
silver  and  less  often  by  a  late  generation  of  chalcopyrite.  The 
latter  mineral  is  frequently  observed  to  grade  off  into  light  yellow 
areas  which  on  examination  with  high  power  are  found  to  con- 
sist of  an  exceedingly  fine  "  intergrowth "  of  chalcopyrite  and 
argentite  (or  stromeyerite).  The  two  constituents  were  arranged 
in  angular  outline  too  intricate  to  admit  of  reproduction.  This  is 
believed  to  be  a  replacement  phenomenon,  since  the  grading  off 
occurs  on  borders  of  chalcopyrite  masses. 


322  F.  N.  GUILD. 

An  intergrow  th  of  argentite  and  stromeyerite  identical  in  ap- 
pearance with  that  of  stromeyerite  and  galena  (Plate  XIV., 
A),  except  that  the  argentite  areas  appeared  duller  than  the 
corresponding  galena,  has  been  observed  in  material  from  Mt. 
Lyell,  Tasmania.  This,  as  explained  before,  is  thought  to  repre- 
sent some  sort  of  equilibrium  in  areas  where  there  is  more  of  the 
argentite  molecule  than  is  necessary  to  form  stromeyerite. 

Native  Silver. — Native  silver  is  believed  to  have  ordinarily 
been  formed  by  the  alteration  of  the  earlier  silver  minerals, 
mainly  argentite.  It  has  also  frequently  been  described  as  coat- 
ing the  ruby  silvers,  polybasite,  etc.  It  is  said  to  be  "  primary  " 
in  the  zeolitic  copper  ores  of  Lake  Superior.'*'  It  sometimes  per- 
sists to  great  depths  as  in  the  Aspen  mining  district  of  Colorado, 
where  it  is  found  crossing  barite  900  feet  below  the  surface.^^ 
At  Creede,  Colorado,  it  is  found  1,200  feet  below  the  surface.'*^ 
It  has  generally  been  assumed  that  silver  is  typically  a  supergene 
mineral,  yet  the  compounds  of  silver  are  known  to  be  weak  and 
should  therefore  easily  break  down  under  a  variety  of  conditions. 
As  early  as  1843  Bischof  found  that  superheated  steam  was 
capable  of  reducing  argentite  to  metallic  silver,  which  appeared 
in  arborescent  shapes  very  much  as  in  nature.  Other  minerals  of 
silver  treated  in  the  same  way  yielded  similar  results. ^'^  These 
experiments  have  been  repeated  from  time  to  time  and  modified, 
Moesta  even  finding  that  the  reaction  could  take  place  at  100  de- 
grees.^^  From  considerations  of  this  kind,  coupled  with  geo- 
logical data,  Kato  thinks  that  native  silver  in  certain  ore  deposits 
of  Japan  was  deposited  by  hypogene  solutions. ^^     From  Vogt's 

"  Emmons,  "  The  Enrichment  of  Sulphide  Ores,"  Bui.  U.  S.  Geol.  Surv., 
No.  529,  p.  118,  1913. 

^s  Lindgren,  quoted  from  oral  communication  in  Emmons,  "  The  Enrich- 
ment of  Sulphide  Ores,"  Bui.  U.  S.  Geol.  Surv.,  No.  529,  p.  118,  1913. 

4^  Emmons,  loc.  cit.,  p.  119. 

50  Bischof,  "  Einige  Bemerkungen  iiber  die  Bildung  der  Gangmasses,"  Pogg. 
Ann.,  60,  p.  285. 

5^  Quoted  by  Vogt  in  his  paper :  "  Ueber  die  Bildung  des  gediegenen  Silbers, 
besonders  des  Kongsberger  Silbers,  durch  Secondarprocesse  aus  Silberglanz 
und  andercn  Silbererzen,"  Zeit.  prakt.  Geol.,  Vol.  7,  p.  113,  1899. 

°2  T.  Kato,  "The  Ore  Dejosits  in  the  Environs  of  Hanano-Yama,  near  the 
town  of  Oda,  province  of  Nagato,  Japan,"  Meiji  College  of  Technology  Journ., 
Vol.  I,  No.  I,  p.  32. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  323 

article  on  native  silver  at  Konsberg  one  might  vv^ell  arrive  at  a 
similar  conclusion  concerning  the  paragenesis  of  some  of  the 
silver  at  that  locality.  He  states  that  "  die  Reduction  des  Silber- 
glanzes  zu  metallischen  Silber  geschah  hauptsachlich  schon  friiher 
als  die  Bildung  des  Kalkesphates  und  des  Flussspathes,  des  Mag- 
netkies  und  des  jiingeren  Schvvefelkies,  also  ungefahr  gleich  nach 
der  Auskrystallization  des  Silberglanzes." 

In  the  investigation  of  the  silver  minerals  cases  have  been  ob- 
served where  the  complex  silver  minerals  have  broken  down  into 
native  silver.  The  occurrence  of  native  silver  in  the  form  of 
thousands  of  microscopic  specks  in  a  specimen  of  stromeyerite 
from  Tombstone,  Arizona,  as  well  as  a  similar  occurrence  in 
argentite  from  Butte,  Montana,  has  already  been  mentioned. 
This  is  the  first  step  in  the  breakdown  of  the  earlier  silver  min- 
eral, solution,  transportation  and  recrystallization  being  necessary 
for  the  production  of  the  delicate  structure  so  frequently  ob- 
served. In  Plate  XVII.,  B,  is  seen  illustrated  a  veinlet  of  silver 
in  an  argentite  specimen  from  Butte,  Mont.  The  same  specimen 
showed  rough  masses  of  native  silver  around  the  borders  of  the 
argentite  areas.  A  somewhat  similar  veinlet  is  seen  in  XII.,  C, 
filling  a  crack  in  stromeyerite  and  associated  minerals,  and  ex- 
panding into  a  mass  of  considerable  size.  Plate  XVII.,  C,  shows 
the  characteristic  manner  in  which  stromeyerite  breaks  down  into 
native  silver  as  observed  in  the  specimens  from  the  Silver  King 
mine,  near  Globe,  Arizona.  The  silver  is  arranged  in  beautiful 
filiform  structure,  the  branches  of  which  envelop  individual 
chalcocite  grains,  some  of  the  finer  filaments  even  extending  into 
fracture  and  cleavage  cracks  of  the  chalcocite,  thus  completing 
the  intricate  design.  Occasionahy  areas  are  found  where  the 
whole  design  is  roughly  oriented  with  reference  to  cleavage  di- 
rections of  chalcocite.  All  of  these  features  are  well  brought 
out  by  etching  with  potassium  cyanide  solution,  when  the  outline 
of  the  individual  grains,  as  well  as  their  cleavage,  is  made  clearer. 
The  areas  showing  the  structure  described  grade  into  stromeyer- 
ite, when  the  native  silver  disappears  altogether,  or  is  confined 
to  borders,  veinlets  or  clumps  of  more  or  less  rounded  outline 
(Plate  XIL,  C).    The  causes  which  have  been  responsible  for  the 


324  F.  N.  GUILD. 

filiform  structure  now  become  clear.  Stromeyerite  has  broken 
down  into  chalcocite  and  native  silver.  The  chalcocite  has  crys- 
tallized into  definite  grains  of  varying  size.  The  silver  in  recrys- 
tallizing  has  formed  around  these  grains,  extending  everywhere 
into  the  minutest  cracks.  As  would  naturally  be  expected,  the 
silver  is  also  found  crystallizing  in  cracks  and  along  borders  of 
other  minerals  both  gangue  and  ore.  Palmer  and  Bastin  have 
shown  that  many  metallic  ores,  among  them  chalcocite,  precipitate 
in  the  laboratory  free  silver  from  a  silver  sulphate  solution.^^ 
This  action  may,  in  a  measure,  contribute  to  the  development  of 
the  filiform  structure,  a  molecule  of  chalcocite  always  being  at 
hand  to  precipitate  silver  should  a  soluble  salt  be  formed  from 
the  argentite  portion  of  the  double  molecule. 

Native  silver  resulting  from  the  breakdown  of  ruby  silver  has 
already  been  mentioned  and  is  illustrated  in  Plate  XIV.,  C  (Du- 
rango,  ]\Iexico).  Plate  XVII.,  D  shows  spots  of  native  silver 
in  a  veinlet  of  tetrahedrite  crossing  smaltite  from  the  Cobalt  dis- 
trict, Ontario.  In  other  specimens  from  the  same  locality  con- 
taining more  massive  tetrahedrite  (freibergite),  numerous 
bunches  of  mossy  silver  were  observed  with  their  spaces  filled 
with  an  intimate  mixture  of  pyrargyrite  and  proustite.  Here  the 
tetrahedrite  had  first  broken  down  into  the  ruby  silvers,  which 
still  later  yielded  native  silver. 

Other  specimens  have  been  examined,  in  which  the  mineral  as- 
sociated with  the  native  silver  and  apparently,  at  least,  replaced 
by  it,  has  not  been  its  source.  In  such  cases  the  associated  min- 
eral may  have  been  a  precipitant  for  silver-bearing  solutions,  or 
it  may  have  been  itself  earlier  replaced  by  a  silver  mineral,  which 
later  broke  down  completely  into  the  native  metal.  Character- 
istic examples  of  the  first  case  are  to  be  observed  in  specimens  of 
cobalt-silver  ores  from  the  Cobalt  district,  Canada,  illustrated  in 
Plate  XVIII. ,  A  and  B.  Here  veinlets  of  native  silver  are  seen  re- 
placing smaltite  and  niccolite.  In  Plate  XVIII.,  A,  the  large  silver 
vein  is  found  to  have  followed  up  a  niccolite  vein,  which  had 
earlier  formed  by  replacement  of  smaltite.     In  other  portions  of 

53  Palmer   and   Bastin,   "  Metallic  Minerals  as   Precipitants   of   Silver  and 
Gold,"  Ecox.  Geol.,  Vol.  8,  p.  140,  1913. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  325 

the  same  specimen  silver  is  found  in  veinlets  crossing  niccolite 
areas.  This  association  calls  to  mind  the  researches  of  Palmer, 
who  found  niccolite  to  be  an  especially  good  precipitant  of  silver 
in  silver  sulphate  solutions.^*  Plate  XVIII. ,  B,  presents  some 
features  which  have  not  yet  been  satisfactorily  explained.  These 
are,  first,  the  presence  of  euhedral  crystals  of  smaltite  set  in  a 
groundmass  of  native  silver,  and  second,  the  presence  of  a  central 
mass  of  silver  (or  sometimes  bismuth,  as  illustrated  in  Plate  XX., 
B),  surrounded  by  a  more  or  less  concentric  arrangement  of  the 
cobalt  and  nickel  minerals.  Plate  XVIII.,  B,  is  of  too  high  a  mag- 
nification to  show  this  relation  to  the  best  advantage.  It  is  better 
illustrated  for  bismuth  in  Plate  XX.,  B.  The  silver  and  bismuth 
are  without  doubt  later  than  the  associated  minerals,  as  illus- 
trated by  vein  relations,  yet  they  are  characteristically  found  in  the 
centers  of  spherical  or  botryoidal  masses.  This  structure  will 
again  be  referred  to  in  the  discussion  of  the  associated  cobalt  and 
nickel  minerals. 

Plate  XVIII.,  C  and  D,  show  the  replacement  of  bornite  by  native 
silver  possibly  via  stromeyerite.  The  specimen  in  which  these 
structures  were  observed  was  also  from  the  Cobalt  district.  Plate 
XVIII. ,  D,  resembles  some  of  the  so-called  graphic  intergrowth 
and,  as  shown  by  transitional  types,  is  a  replacement  phenomenon. 
Other  specimens  from  Cobalt  show  stromeyerite  replacing  bornite. 
This  calls  to  mind  a  somewhat  similar  feature  observed  in  ma- 
terial from  the  Silver  King  mine,  Arizona,  in  which  stromeyerite 
has  replaced  bornite  in  a  rough  graphic  design  (Plate  XII.,  D). 
A  further  replacement  of  stromeyerite  by  silver  would  develop 
a  structure  similar  to  that  shown  in  Plate  XVIII.,  A. 

Plate  XIX.,  A  and  B,  illustrate  a  peculiar  dendritic  structure  of 
smaltite  and  silver  in  calcite  from  Cobalt,  Ontario.  In  Plate  XIX., 
B,  it  will  be  seen  that  the  smaltite  has  the  same  shell-like  form 
enclosing  native  silver  as  is  so  frequently  in  these  ores.  An  ex- 
amination of  the  borders  between  smaltite  and  silver  seems  to 
indicate  that  a  replacement  of  smaltite  by  silver  has  taken  place. 
The  dendritic  structure  then  becomes  a  phenomenon  of  smaltite, 
silver  solutions  having  later  replaced  the  cores  of  the  regularly 

54  Palmer,  "  Studies  in  Silver  Enrichment,"  Econ.  Geol.,  Vol.  9,  p.  664,  1914. 


326  F.  N.  GUILD. 

arranged  smaltite  grains.  Shells  of  smaltite  enclosing  gangue 
are  illustrated  in  Plate  XX.,  D.  While  these  shells  do  not  show 
dendritic  grouping,  a  replacement  of  the  gangue  in  the  centers 
would  develop  a  structure  similar  to  that  seen  in  individual  grains 
or  shells  illustrated  in  Plate  XIX.,  B. 

Cerargyrite. — Although  native  silver  is  the  natural  end  product 
in  the  transformation  of  the  various  silver  minerals,  it  may  itself 
be  the  object  of  attack  of  supergene  solutions  containing  chlorids 
or  hydrogen  sulphide.  Its  alteration  to  cerargyrite  has  been  ob- 
served in  specimens  from  the  Stonewall  Jackson  mine,  near  Pres- 
cott,  Arizona,  and  is  illustrated  in  Plate  XIX.,  C.  The  gangue  is 
siderite  and  the  silver  occurs  embedded  in  it  in  rough  grains 
showing  all  grades  of  alteration  to  the  chloride.  Argentite  is 
also  present  in  small  grains  and  may  have  been  the  source  of  the 
free  silver. 

Vogt  has  observed  the  breakdown  of  argentite  into  native 
silver,  which  in  its  turn  has  formed  argentite  again  partly  by  the 
action  of  hydrogen  sulphide  formed  from  decaying  mine  timbers 
and  blasting  powder. ^^ 

Hiintilite,  Ag^As?  and  Dyscrasite,  Ag.Sb  to  Ag^Sb. — Only 
one  specimen  of  so-called  huntilite  from  the  original  locality, 
Silver  Islet,  Lake  Superior,  was  examined.  It  consisted  of  fine 
dendritic  replacements  in  dolomite,  which  under  the  reflecting 
microscope  were  found  to  be  made  up  of  a  central  portion  of  a 
cream  mineral  like  dycrasite,  surrounded  by  rims  of  native  ar- 
senic. The  occurrence  is  illustrated  in  Plate  XIX.,  D.  The  cen- 
tral portion  (huntilite  ?)  became  slightly  darkened  on  etching 
with  nitric  acid  thus  giving  sufficient  contrast  to  make  the  photo- 
graph possible. 

One  specimen  of  dyscrasite  from  the  Cobalt  district,  Ont.,  was 
studied.  Under  high  power  it  showed  in  places  a  faint  compli- 
cated structure  perhaps  due  to  unmixing  of  a  solid  solution.  This 
was  brought  out  more  strikingly  on  etching  with  nitric  acid  which 
yielded  rather  definite  grains  of  etched  material  closely  crowded 

5s  Vogt,  "  Ueber  die  Bildung  des  gediegenen  Silbers,  besonders  des  Kongs- 
berger  Silbers,  durch  Secondarprocesse  aus  Silberglanz  und  anderen  Silberer- 
zen,"  Zeit.  prakt.  Geo!.,  Vol.  7,  p.  113,  1899. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  327 

together  and  set  in  a  background  of  less  affected  material.  The 
mass  was  associated  with  smaltite  into  which  extended  small  vein- 
lets  of  dyscrasite. 

Brongniardite,  PhS,  Ag^S,  Sh2Sz- — This  mineral  was  discov- 
ered by  Damour  in  1849.^^  Only  massive  material  has  been 
found,  the  isometric  crystals  described  by  Damour  in  1854^'^ 
having  been  proved  by  Prior  and  Spencer  to  be  the  entirely  dif- 
ferent species,  stanniferous  argyrodite.^^  Murdoch  finds  some 
specimens  to  appear  to  be  a  mixture  of  ruby  silver,  galena(?), 
and  possibly  miargyrite.  Others  show  a  distinct  mineral  close  to 
argentiferous  jamesonite.^^ 

One  specimen  of  brongniardite  from  Chococomete,  Bolivia, 
was  examined  by  means  of  the  reflecting  microscope.  It  was 
found  to  show  a  structure  almost  identical  in  appearance  to  the 
blue  and  white  effect  in  some  chalcocite.  The  pattern  appeared 
in  some  places  in  patchy  designs,  in  others  in  parallel  bands  call- 
ing to  mind  the  poly  synthetic  twinning  of  the  feldspars.  In 
places  there  were  two  series  of  bands  crossing  at  an  angle.  The 
bluish  streaks  were  found  to  be  pyrargyrite  as  suggested  by  Mur- 
doch. When  examined  in  fragments  this  portion  was  shown  to 
be  transparent  on  thin  edges  and  to  have  a  deep  cherry  red  color. 
These  streaks  were  also  found  to  be  rapidly  darkened  when  ex- 
posed to  the  unscreened  electric  arc.  This,  as  mentioned  before, 
is  a  peculiar  effect  produced  on  certain  of  the  silver  minerals 
notably  pyrargyrite  and  argentite.  A  polished  surface  of  brong- 
niardite etched  in  this  manner  is  illustrated  in  Plate  XX.,  ^4.  The 
pyrargyrite  streaks  show  a  darkening  which  begins  to  appear  in 
round  dots  at  first  isolated  but  soon  rapidly  increasing  in  number 
and  size  until  the  whole  area  is  blackened.  These  investigations 
show  that  brongniardite,  while  possibly  a  distinct  species  under 

56  Damour,  "  Sur  la  brongniardite,  nouvelle  espece  minerale,"  Ann.  de 
Mines  (IV.),  XVI.,  p.  227. 

^"^  Damour,  "  Sur  la  crystallization  de  la  brongniardite,  espece  minerale," 
Ann.  de  Mines  (V.),  VI.,  p.  146. 

5S  Prior  and  Spencer,  "  Stanniferous  Argyrodife  from  Bolivia,"  Mineralog- 
ical  Mag.,  Vol.  12,  p.  5. 

^9  Murdoch,  "  Microscopic  Determination  of  the  Opaque  Alinerals,"  p.  37, 
1016. 


32  8  F.  N.  GUILD. 

the  special  conditions  under  which  it  was  formed,  has  under  the 
present  conditions  broken  down  into  pyrargyrite  and  some  other 
constituent  or  constituents  not  yet  identified. 

Schirmerite,  siAg^,  Pb)S,  2Bi2S^. — Only  one  specimen  of  this 
mineral  was  investigated.  It  was  from  Geneva,  Clear  Creek  Co., 
Colo.,  and  represented  cavity  filling  in  quartz.  It  was  associated 
with  galena,  chalcopyrite  and  covellite.  The  material  was  so 
scant  that  with  the  exception  of  covellite  the  relations  could  not 
be  made  out.  Covellite  appeared  as  borders  and  veinlets  replacing 
the  schirmerite. 

V.    THE  ASSOCIATED  COPPER  MINERALS. 

Chalcopyrite,  Bornite,  Chalcocite  and  Covellite. 

Chalcopyrite. — Chalcopyrite  is  a  very  common  associate  of  the 
silver  minerals  in  many  deposits  while  it  is  practically  absent  in 
others.  In  deposits  rich  in  the  copper  minerals  it  plays  the  same 
role  in  replacement  processes  as  has  been  found  to  be  the  case  in 
typical  copper  veins.  It  may  be  either  a  hypogene  or  supergene 
mineral.  As  a  hypogene  mineral  it  is  very  typically  found  as 
minute  dots  and  dashes  either  crystallographically  arranged  or 
scattered  irregularly  throughout  sphalerite  areas  (X.,  C  and  XVI., 
B).  The  dots  sometimes  enlarge  into  more  or  less  vein-like 
shapes  and  are  thought  to  be  due  to  an  early  replacement  of 
sphalerite  by  chalcopyrite.  It  is  further  found  as  a  hypogene 
mineral  replacing  pyrite  in  the  form  of  veinlets  and  borders 
which  frequently  expand  into  areas  of  considerable  extent.  These 
areas,  when  associated  with  galena,  are  invariably  found  to  be 
earlier  than  galena,  also  a  hypogene  mineral.  Thus  in  Plate  X., 
D,  chalcopyrite  is  seen  to  have  been  replaced  by  galena. 

Supergene  chalcopyrite  or  at  least  chalcopyrite  of  a  very  late 
generation  has  been  observed  in  many  polished  specimens  of  the 
silver  ores.  It  occurs  as  borders  and  veinlets  in  galena  and  the 
late  silver  minerals.  Thus  in  a  specimen  of  argentiferous  galena 
from  Rimini,  Mont.,  zigzag  veinlets  were  observed  following 
cleavage  directions  of  galena.  In  Plate  XVI.,  D,  already  described 
under  argentite,  it  appears  replacing  that  sulphide  as  a  delicate 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  329 

fringe  along  the  borders.  It  is  also  observed  replacing  bornite,  as 
described  by  Graton  and  Murdoch^°  and  Tolman,*^^  in  the  form  of 
sharp  blades  often  oriented  in  symmetrical  design. 

The  occurrence  of  two  generations  of  chalcopyrite  in  the  silver 
deposits  appears  to  be  a  very  characteristic  feature,  one  genera- 
tion appearing  just  before  galena,  and  the  other  later  than  galena 
or  even  later  than  the  rich  silver  minerals. 

In  specimens  from  the  Cobalt  district,  Ont.,  chalcopyrite  was 
observed  only  in  traces  where  it  appeared  as  the  late  generation 
crossing  argentite  and  extending  into  the  associated  smaltite  (in 
specimen  illustrated  in  Plate  XVII.,  A). 

Plate  XIII. ,  A,  already  described,  illustrates  a  so-called  inter- 
growth  of  chalcopyrite  and  stromeyerite  from  Mt.  Lyell,  Tas- 
mania. Here  chalcopyrite  is  believed  to  be  hypogene. 
■  Bornite. — Bornite  is  a  comparatively  rare  associate  of  the  silver 
minerals,  being  found  only  in  the  copper-silver  type  of  deposits, 
as  illustrated  in  the  specimens  described  from  Silver  King,  Ari- 
zona (Plate  XII.,  C),  and  the  Mt.  Lyell  district,  Tasmania.  It 
then  generally  shows  the  same  characteristics  as  appear  in  the 
ordinary  copper  deposits,  stromeyerite  sometimes  taking  the  place 
of  chalcocite  in  the  series  of  replacements.  An  unusual  occur- 
rence with  native  silver  has  been  observed  from  the  Cobalt  dis- 
trict, Ont.,  and  is  illustrated  in  Plate  XVIII.,  C  and  D. 

When  associated  with  stromeyerite  and  other  silver  minerals  it 
has  sometimes  been  observed  to  show  anomalous  features.  In 
specimens  from  Silver  King,  Ariz.,  remnants  were  found  showing 
unusual  colors.  On  other  residual  grains  this  peculiarity  appears 
as  borders  fading  off  almost  imperceptibly  into  stromeyerite.  It 
was  at  first  thought  that  these  effects  might  be  due  to  solid  solu- 
tions of  stromeyerite  and  bornite,  but  on  examination  with  high 
power  the  borders  were  proved  to  be  non-homogeneous,  but  of 
such  intricate  structure  as  not  to  permit  of  photographing.  The 
fading-off  tints  are  due  then  to  different  relative  amounts  of  the 
two  minerals  in  a  sort  of  sub-microscopic  intergrowth,  which  is 

6°  Graton  and  Murdoch,  "  The  Sulphide  Ores  of  Copper,"  Trans.  Am.  Inst. 
Min.  Eng.,  Vol.  45,  p.  74,  1913. 

^1  Tolman,  "  Observation  on  Certain  Types  of  Chalcocite  and  their  Charac- 
teristic Etch  Patterns,"  Trans.  Am.  Inst.  Min.  Eng.,  Vol.  52,  p.  401,  1916. 


330  F.  N.  GUILD. 

simply  a  detail  in  the  process  of  replacement.  Only  in  the  size 
of  the  particles  does  it  differ  materially  from  the  so-called  inter- 
growths  so  frequently  described. 

Chalcocite  and  Covellite. — Chalcocite  is  also  far  less  common 
as  an  associate  of  the  silver  minerals  than  is  chalcopyrite.  When 
present  it  owes  its  origin  to  the  same  conditions  which  give  rise 
to  the  various  enrichment  products  of  the  typical  copper  deposits. 
Thus  at  Silver  King,  Arizona,  it  is  abundant  as  one  of  the  prod- 
ucts formed  from  metasomatic  processes  in  the  lean  pyritic  ores. 
Here  it  occurs  in  two  generations,  possibly  representing  depo- 
sition from  hypogene  and  supergene  solutions.  As  a  representa- 
tive of  the  first  mentioned  type,  it  is  found  in  rather  large  areas 
showing  good  cleavage  in  three  directions  when  etched  with 
potassium  cyanide,  and  rich  in  the  blue  and  white  effects.  The 
other  type  is  shown  in  thin  veinlets,  often  associated  with  covel- 
lite and  other  oxidation  products  crossing  pyrite,  chalcopyrite 
and  bornite.  Chalcocite  resulting  from  the  breakdown  of  stro- 
meyerite  by  which  native  silver  is  also  produced,  has  already 
been  described  (Plate  XVII.,  C).  As  explained  before  the  pos- 
sibility of  this  breakdown  taking  place  in  hypogene  solutions  is 
not  excluded. 

Covellite,  as  suggested  above,  is  often  found  in  small  quan- 
tities in  silver  ores,  where  it  is  believed  to  represent  deposition 
from  supergene  solutions.  It  is  most  frequently  associated  with 
malachite  in  fine  veinlets  crossing  nearly  all  of  the  earlier  sul- 
phides, but  more  especially  sphalerite,  thus  recalling  the  first 
synthesis  of  covellite  under  geo-chemical  conditions. ^^ 

VI.    THE  ASSOCIATED  COBALT  AND  NICKEL  MINERALS. 

These  minerals  may  all  appropriately  be  considered  in  one  group, 
since  they  are  always  intimately  associated  with  one  another  and 
even  "  intergrown  "  in  such  a  manner  as  to  make  separate  treat- 
ment impossible. 

The  predominant  nickel  mineral  is  later  than  smaltite,  as  is 
shown  in  Plate  XVIII.,  A,  where  a  veinlet  of  niccolite  crosses  the 

62  Rogers,  "A  New  Synthesis  and  New  Occurrence  of  Covellite,"  Sc.  of 
Mines  Quar.,  Vol.  Z'^,  p.  298,  1910-11. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  331 

cobalt  mineral.  The  photograph  also  shows  native  silver  to  be 
still  later.  Relationships  not  so  easy  to  interpret,  however,  are 
observed  in  Plate  XX.,  B.  Here  the  minerals  are  arranged  in  the 
spherical  or  concentric  manner  so  often  observed  in  these  ores  and 
so  difficult  of  explanation.  The  outer  shell  consists  of  smaltite; 
this  is  followed  by  an  intricate  intergrowth  of  smaltite  and  nic- 
colite,  then  a  layer  of  distinct  niccolite  grains,  while  the  center  is 
occupied  by  native  bismuth.  All' of  these  minerals  are  of  high 
luster,  so  that  etching  with  nitric  acid  was  found  necessary  to 
bring  out  the  structure  for  photographing.  The  complexity  of 
the  structure  is  sometimes  increased  by  the  presence  of  breithaupt- 
ite  (NiSb)  intergrown  with  the  cobalt  and  nickel  minerals,  as 
well  as  native  silver,  which  tends  to  occupy  the  central  portions 
and  also  to  enter  the  intergrowths.  Even  the  apparently  pure 
smaltite  areas  seem  to  be  made  up  of  different  species,  as  there  is 
a  slight  difference  of  color  among  the  various  grains,  and  con- 
siderable variation  in  hardness  as  shown  by  relief.  Chemical 
analyses  have  also  proved  that  this  mineral,  aside  from  containing 
varying  amounts  of  nickel  (chloanthite),  shows  a  ratio,  R:As, 
varying  from  1:2  to  nearly  1:3,  thus  sometimes  approaching 
skutterudite  (RAss).^^ 

The  concentric  structure  has  been  explained  by  Campbell  and 
Knight  as  due  to  cavity  filling.  Then,  of  course,  the  smaltite 
shell  is  first  deposited  on  the  interior  surface  of  the  cavity,  the 
center  being  filled  with  the  later  minerals.*'^  The  spherical  masses 
are  grouped  together  in  great  complexity,  in  fact  in  a  manner  that 
seems  to  exclude  the  probability  of  their  ever  having  been  cavities. 
Smaltite  is  frequently  found  in  calcite  without  the  associated 
minerals  mentioned  above ;  then  it  shows  a  strong  tendency  to 
occur  in  concentric  masses  (Plate  XX.,  C)  or  shell-like  forms 
(Plate  XX.,  D),  the  centers  of  which  are  also  made  up  of  calcite. 
The  study  of  thin  sections  of  this  material  shows  that  this  is  a  re- 
placement phenomenon,  the  unusual  shapes  being  due  to  a  peculiar 

63  Dana,  "  System  of  Mineralogy,"  6th  ed.,  p.  88. 

6*  Campbell  and  Knight,  "  A  Microscopic  Examination  of  the  Cobalt-nickel 
Arsenides  and  Silver  Deposits  of  Temiskaming,"  EcoN.  Geol.,  Vol.  i,  p.  767, 
1906. 


332  F.  N.  GUILD. 

habit  of  smaltite  not  yet  understood.  The  whole  calcite  filling  is 
made  up  of  one  individual  and  that  is  continuous  with  the  calcite 
outside  the  shell  as  shown  by  identity  in  extinction  angle,  and  con- 
tinuance of  such  features  as  twinning  bands  and  parting  planes. 
This  shows  conclusively  that  smaltite  has  come  in  later  than  the 
crystallization  of  the  calcite.  The  continued  replacement  of 
calcite  by  smaltite  with  the  entrance  of  the  later  minerals,  nic- 
colite  and  bismuth,  might  develop  a  structure  as  appears  in  Plate 
XX.,  B,  since  the  later  arrivals,  finding  the  spaces  between  the 
shells  already  occupied  by  smaltite,  and  showing  a  preference  for 
calcite,  would  find  it  imperative  to  replace  remnants  of  that  min- 
eral found  inside  the  shell.  Thus  the  early  fonned  shells  of 
smaltite  protect  the  calcite  centers,  so  that  they  are  the  last  to  be 
attacked.  I  f  this  explanation  is  correct  we  should  expect  to  find  nic- 
■colite  and  the  later  minerals  sometimes  outside  the  shell  for  such 
cases  as  represented  an  incomplete  earlier  replacement  of  that 
calcite  by  smaltite.  This  arrangement  has  actually  been  observed 
in  several  specimens.  A  structure  like  that  appearing  in  Plate 
XX.,  C  would,  on  later  replacement  of  calcite  by  niccolite,  show 
several  alternate  bands  of  niccolite  and  smaltite,  actual  occur- 
rences of  which  have  also  been  noted.  The  intergrowths  might 
easily  have  resulted  from  a  replacement  of  smaltite  by  niccolite, 
or  even  by  the  replacement  of  residual  calcite  forming  the  ragged 
borders  of  some  of  the  smaltite  rings  (Plate  XX.,  C). 

The  obscure  habit  of  smaltite  to  assume  shell-like  forms  is 
probably  connected  with  botryoidal  structure,  as  shown  by  the 
fact  that  typical  botryoidal  surfaces  are  frequently  observed  on 
breaking  specimens. 

Plate  XXL,  B^  shows  another  slight  variation  in  the  spherical 
structure  so  often  assumed  by  smaltite.  A  few  euhedral  crystals 
have  also  developed.  This  specimen  as  well  as  all  of  the  other 
nickel  and  cobalt  minerals  described  above  were  from  the  Cobalt 
district,  Ontario. 

Many  other  interesting  species  are  observed  associated  with  the 
cobalt  ores,  but  their  investigation  would  be  too  remote  from  the 
purposes  of  this  paper. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  333 

VII.    THE  GANGUE  MINERALS. 

The  fact  that  gangue  minerals  must  be  investigated  by  different 
methods  from  those  employed  for  opaque  ores  is  perhaps  respon- . 
sible  for  the  notion  sometimes  implied  that  these  constitute  two 
distinct  classes  of  minerals.  The  gangue  minerals  form  no  in- 
significant part  of  vein  phenomena,  and  their  position  in  the  se- 
quence of  deposition  frequently  throws  light  upon  other  important 
questions  of  paragenesis.  Thus  it  is  held  by  Rogers  that  sericite 
and  chlorite  are  later  than  hypogene  chalcocite  and  earlier  than 
supergene.*'^  If  this  conclusion  is  correct,  as  seems  probable,  the 
relation  of  these  silicates  to  the  ore  minerals  becomes  of  greater 
importance  in  determining  paragenetic  features. 

The  silicates  when  considered  by  themselves  are  found  to  be 
subject  to  a  process  of  breakdown  and  replacement  paraheled  by 
the  replacement  processes  of  the  ore  minerals.  Thus  the  feldspars 
break  down  into  sericite,  kaolin,  carbonates  and  other  products. 
The  resulting  minerals  appear  as  veinlets,  borders,  and  other 
forms  exactly  as  in  the  case  of  the  opaque  minerals.  In  Plate 
XXL,  C,  calcite  is  shown  replacing  quartz.  This  is  taken  from  the 
gangue  of  a  silver  specimen  from  Sandon,  B.  C.  It  is  a  photo- 
graph of  a  polished  specimen  and  is  a  good  illustration  of  the  fact 
that  the  reflecting  microscope  may  be  a  better  instrument  for 
bringing  out  relationships,  even  of  the  transparent  minerals,  than 
is  the  ordinary  type.  The  phenomenon  of  replacement  in  Plate 
XXL,  C,  is  not  essentially  different  from  that  shown,  for  ex- 
ample, in  Plate  XL,  D,  which  illustrates  the  replacement  of 
sphalerite  by  tetrahedrite. 

The  early  ore  minerals  are  the  ones  most  often  found  replacing 
gangue  constituents,  and  many  cases  might  be  cited  in  which  these 
processes  have  been  described.  The  replacement  is  not  confined, 
however,  to  the  earlier  sulphides  as  shown  in  Plate  XV.,  D,  where 
stephanite  is  seen  to  be  replacing  particles  in  a  brecciated  gangue. 
P3'rite,  however,  an  early  sulphide,  is  also  seen  to  have  taken  part 
in  the  same  kind  of  replacement.  A  similar  case  has  already  been 
cited  from  the  work  of  Fenner  in  which  the  constituents  of 

^5  Rogers,    "  Sericite    a    Low-temperature    Hydrothermal    Mineral,"    EcON. 
Geol.,  Vol.  II,  p.  Ii8,  1916. 


334  F.N.  GUILD. 

rhyolite  porphyry  have  been  replaced  by  chalcopyrite  and  ste- 
phanite.  Argentiferous  galena  is  seen  to  be  replacing  calcite 
along  cleavage  lines  in  Plate  XXL,  D. 

Carbonates  are  particularly  characteristic  as  gangue  minerals 
of  the  silver  ores,  especially  of  the  lead-silver  type.  This  indi- 
cates a  strong  tendency  for  these  ores  to  be  deposited  from  neutral 
or  alkaline  solutions,  since  carbonates  could  not  develop  in  acid 
solutions.  Calcite,  dolomite,  siderite  and  rhodochrosite  have  fre- 
quently been  noted  as  important  associates  of  silver  minerals. 
)(/y  c  Siderite  is  shown  in  Plate  XVI li-..  A-  native  silver  altering  to 
cerargyrite  being  seen  in  the  same  specimen. 

Barite  is  also  a  frequent  gangiie  mineral  in  silver  deposits  and 
has  usually  been  described  as  earlier  than  the  sulphides.  Thus  at 
Aspen,  Colo.,  it  is  cut  by  veinlets  of  argentite  and  native  silver.*^^ 
A  similar  occurrence  was  observed  in  material  from  Sandon, 
B.  C,  in  which  thin  sections  showed  argentite  replacing  barite 
in  fringes  along  the  borders  of  euhedral  crystals. 

VIII.    THE  IDENTIFICATION   OF   SILVER   MINERALS  ON   POLISHED 

SURFACES. 

A  good  summary  of  the  work  thus  far  accomplished  on 
methods  for  the  determination  of  the  opaque  minerals  has  already 
been  given  by  Murdoch.*^'  From  this  it  will  appear  that  very 
little  detailed  study  had  been  made  of  the  silver  minerals.  In 
]\Iurdoch's  tables  they  are  grouped  according  to  color,  hardness, 
and  action  of  various  reagents  as  KCN,  HNO3,  etc.  Argentite, 
polybasite  and  stephanite  are  put  down  as  grayish  white;  silver, 
dyscrasite  and  huntilite  as  creamy  white  and  proustite  and  pyrar- 
g}Tite  as  bluish  white.  His  etching  and  tarnishing  tests  are  not 
characteristic  and  have  been  found  of  little  value  for  purposes  of 
identification. 

As  stated  in  the  introduction  three  important  means  have  been 
employed  for  the  identification  of  the  silver  minerals  discussed 

6^  Spurr,  "  Geologj'  of  tlie  Aspen  Mining  District,"  Alon.  U.  S.  Geol.  Surv., 
Vol.  31,  p.  221,  1898. 

^"  JMurdoch,  "  Microscopic  Determination  of  the  Opaque  Minerals,"  pp.  4-16, 
1916. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  335 

in  this  paper.  First,  the  observation  of  color,  relief  and  habit  as 
observed  on  polished  surfaces ;  second,  the  study  of  minute  frag- 
ments broken  from  the  surface  by  a  sharp  point  and  transferred 
to  a  glass  slip  for  examination  with  an  ordinary  microscope ;  and 
third,  by  micro-chemical  tests  on  small  fragments  secured  in  the 
same  manner.  Etching  with  nitric  acid  has  been  found  valuable 
in  the  study  of  argentiferous  galena  (Plate  XL,  A  and  C),  the 
silver  minerals  being  less  easily  attacked  by  this  reagent  than 
galena;  for  bringing  out  structure  for  purposes  of  photographing 
as  in  huntilite  (Plate  XIX.,  D)  ;  and  for  developing  structure  in 
the  bismuth-cobalt  specimens  (Plate  XX.,  B).  Potassium  cya- 
nide has  been  found  especially  useful  in  bringing  out  the  compli- 
cated structure  of  stromeyerite  and  in  distinguishing  it  from  chal- 
cocite  (Plate  XIII.,  B  and  C). 

Hardness  and  color  are  best  compared  with  galena,  since  that 
mineral  is  the  one  most  commonly  associated  with  the  silver 
minerals.  In  this  respect  the  tables  of  Murdoch  have  been  found 
of  great  value.  All  of  the  silver  minerals  with  the  exception  of 
tetrahedrite  are  softer  than  galena  and  therefore  stand  in  negative 
relief.  Even  tetrahedrite  as  it  becomes  highly  argentiferous  ap- 
proaches galena  in  hardness.  The  normal  varieties  are  consid- 
erably harder.  Proustite  is  probably  the  one  that  can  be  most 
easily  distinguished  by  color  alone.  It  has  a  characteristic  bluish 
tint  rarely  mistaken  for  that  of  other  minerals.  Pyrargyrite,  in 
the  writer's  opinion,  is  not  perceptibly  blue  in  color,  being  prac- 
tically of  the  same  tint  as  some  tetrahedrite  and  polybasite.  It 
can,  however,  readily  be  distinguished  from  tetrahedrite  by  its 
inferior  hardness,  and  from  each  by  chemical  tests  described 
below. 

Tests  on  fragments  are  made  by  breaking  from  the  polished 
surface  minute  quantities  of  the  mineral  by  means  of  a  sharp  awl, 
transferring  to  a  glass  slip  and  examining  with  a  high  power 
rriicroscope.  Argentite  and  stephanite  are  opaque  even  on  the 
thinnest  edges.  Proustite  is  transparent  even  in  the  thicker  por- 
tions, and  is  of  a  beautiful  red  color,  shading  off  into  amber  on 
the  thinnest  edges.  Pyrargyrite  is  transparent  and  of  a  deep 
cherry  red  color  only  on  thin  edges,  the  thicker  portions  even  ap- 


13^  F.  N.  GUILD. 

preaching  opacity.  Polybasite  and  some  specimens  of  tetra- 
hedrite  react  in  the  same  way  as  pyrargyrite  but  the  presence  of 
copper  in  these  minerals  is  a  distinguishing  feaure.  In  making 
these  tests  as  bright  a  hght  as  possible  should  be  used  and  care 
should  be  exercised  in  focusing  carefully,  otherwise  interference 
of  light  on  sharp  edges  of  black  fragments  may  produce  red  tints. 
Proustite  and  pyrargyrite  may  also  be  distinguished  by  their 
streak,  a  test  which  can  also  be  made  on  very  small  fragments  by 
crushing  on  white  paper.  In  securing  the  material  by  means  of  a 
sharp  point,  the  sectility  of  argentite  is  a  sufficient  test  for  dis- 
tinguishing that  mineral. 

Micro-chemical  tests  are  made  by  securing  a  minute  portion  of 
the  powder  in  the  manner  described  above,  transferring  to  a  glass 
slip,  dissolving  in  nitric  acid  by  warming  slightly,  evaporating 
nearly  to  dryness,  taking  up  in  a  drop  of  distilled  water  and  pro- 
ceeding as  outlined  below. 

The  presence  of  silver  is  learned  by  adding  a  drop  of  dilute 
chlorhydric  acid.  The  characteristic  curdy  precipitate  is  satis- 
factory evidence.  Comparative  tests  may  be  made  on  known 
minerals  so  as  to  be  able  to  judge  if  the  silver  be  present  in  large 
or  small  amounts. 

Copper  is  tested  for  by  adding  potassium  ferrocyanide  in  a 
slightly  acid  solution.  The  peculiar  red  precipitate  is  thoroughly 
characteristic  of  this  element  and  may  even  be  seen  in  the  pres- 
ence of  iron  if  the  drop  of  reagent  is  allowed  to  stand  without 
mixing.  Some  phenomenon  of  diffusion  permits  the  two  precipi- 
tates to  be  seen. 

The  two  tests  outlined  above  are  the  ones  most  often  required 
in  distinguishing  the  silver  minerals  and  it  has  been  found  much 
more  satisfactory  to  remove  the  minute  amount  of  material  re- 
quired, than  to  attempt  making  the  test  directly  on  the  surface. 
Moreover  by  this  method  the  specimen  is  much  less  damaged  for 
further  work.  Having  obtained  the  material  in  solution  as  out- 
lined above,  tests  ma:y  be  made  for  other  elements  according  to 
the  methods  outlined  by  Chamot  for  general  micro-chemical 
work.*'^  In  the  investigation  of  the  cobalt-nickel  minerals  the 
•8  Chamot,  "  Elementary  Chemical  Microscopy,"  1915. 


MICROSCOPIC  STUDY  OF  SILVER  ORES. 


337 


dimethylglyoxime  test  for  nickel  has  been  found  to  be  especially 
useful.  This  is  made  by  making  the  solution  alkaline,  placing  a 
minute  crystal  of  the  oxime  in  the  solution,  covering  with  a  cover- 
glass  and  examining  after  allowing  to  stand  for  some  time.  By 
using  the  solid  reagent  the  red  needles  of  the  nickel  salt  collect 
around  the  crystal  and  the  test  is  believed  to  be  more  delicate. 
The  test  is  satisfactory  in  the  presence  of  large  amounts  of  iron, 
the  needles  easily  being  seen  mingled  with  the  hydroxid.  Thus  a 
specimen  of  socalled  nickeliferous  pyrrhotite,  said  to  contain  not 
more  than  three  per  cent,  nickel,  responded  satisfactorily  to  the 
test. 

In  the  table  below  only  tests  are  listed  that  have  been  found 
to  be  particularly  useful  in  distinguishing  the  common  silver 
minerals.    Those  most  typical  are  marked  with  a  star. 


Argentite  .  . . 

Polybasite  . . 

Pyrargyrite  . 

Stephanite . . 

Tetrahedrite 

Proustite  .  .  . 
Stromeyerite 


Color. 


Grayish. 


Fragments. 


Opaque. 

Red  on  edges.* 
It     I.       II    ^ 

Opaque.* 

Sometimes 

red  on  edges. 
Red* 


Bluish.* 
Purplish  with  Opaque, 
chalcocite.*! 


Hardness. 


Chem.  Tests. 


Less  than 

galena. 
Less  than 

galena. 
Less  than 

galena. 
Less  than 

galena. 
Greater  than 

galena.* 
Less  than  Gn. 
=  Gn. 


Others. 


;Ag. 

Ag*  and  Cu.* 

^Ag. 

Ag. 

Cu*  and  (Ag.) 

Ag. 

sCu  and  Ag.* 


Sectile.* 


Purple  red 
streak. 


Red  streak. 


Stephanite  and  argentite  are  both  opaque,  but  argentite  may 
be  distinguished  by  its  sectility. 

Pyrargyrite  and  polybasite  are  each  red  on  thin  edges,  but 
polybasite  reacts  for  copper. 

Polybasite  and  tetrahedrite  may  each  be  red  on  thin  edges, 
each  react  for  copper,  but  tetrahedrite  is  easily  distinguished  by 
its  greater  hardness. 

A  delicate  test  for  arsenic  and  antimony  so  as  to  distinguish 
between  tennantite  and  tetrahedrite,  as  well  as  between  poly- 
basite and  pearceite,  is  greatly  needed.  Berg  recommends  for 
the  sulfo-salt  minerals  the  following  test:  Dissolve  a  fragment 


33^  F.  N.  GUILD. 

in  potassium  hydroxid  solution,  then  add  chlorhydric  acid. 
Arsenic  if  present  is  throAvn  down  as  the  lemon  yellow  sulphide. 
The  corresponding  antimony  minerals  give  an  orange  precipi- 
tate.^^ While  this  works  well  for  pure  compounds,  it  has  been 
found  to  be  of  little  value  in  studying  the  complex  mixture  ob- 
served in  this  class  of  minerals. 

IX.    RESUME. 

1.  Tetrahedrite  and  argentiferous  galena  are  the  early  sources 
of  the  rich  silver  minerals.  Much  of  this  galena  has  replaced 
tetrahedrite,  so  the  latter  mineral  becomes  the  more  important  of 
the  two. 

2.  The  minerals  antedating  the  deposition  of  silver  in  the  ore 
body  are,  in  the  order  of  sequence,  arsenopyrite,  pyrite  and 
sphalerite. 

3.  Graphic  structures  are  frequently  observed  among  the  silver 
minerals  and  their  associates.  Those  described  are  of  two  classes : 
First,  those  of  undoubted  replacement,  and  second,  those  of  un- 
proved origin  but  thought  to  be  due  to  simultaneous  deposition 
with  later  crystallization  and  segregation  after  graphic  designs. 
To  the  first  class  belong  the  following  so-called  intergrowths : 
Stromeyerite  and  chalcopyrite  (Plate  XIII.,  ^)  ;  bornite  and  stro- 
meyerite  (Plate  XII.,  D)  ;  and  bornite  and  native  silver  (Plate 
XVIII.,  D).  To  the  second  class  belong  the  more  intricate  inter- 
growths between  the  following  minerals :  Stromeyerite  and  chal- 
cocite  (Plate  XIII.,  B  and  C)  ;  stromeyerite  and  galena  (Plate 
XIV.,  ^)  ;  stromeyerite  and  argentite ;  galena  and  proustite  (Plate 
XV.,  B)  ;  and  pyrargyrite  and  some  mineral  not  yet  identified  (in 
socalled  brongniardite  (Plate  XX.,  A)). 

4.  Galena  which  does  not  show  abundant  evidence  of  later  ad- 
ditions or  enrichments  has  not  been  observed  to  contain  more  than 
0.35  per  cent,  silver,  the  average  of  fifteen  specimens  from  rich 
silver-lead  deposits  being  0.20  per  cent.  This  may  be  present  in 
solid  solution  or  sub-microscopic  particles  up  to  nearly  o.io  per 
cent.    Above  that  amount  it  appears  as  spots  of  definite  minerals 

69  Berg,  "  Alikroskopische  Untersuchung  der  Erzlagerstatten,"  p.  46. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  339 

identified  as  tetrahedrite  or  argentite  or  both.  Specimens  of 
galena  with  more  than  about  0.35  per  cent,  silver  show  evidence 
of  later  addition  of  ruby  silver  or  other  rich  silver  minerals  in  the 
form  of  veinlets. 

5.  Whether  the  "enrichment"  of  the  silver  ores  is  due  to 
ascending  or  descending  solutions  has  not  been  decided.  Prob- 
ably both  factors  are  active.  The  writer  is  inclined  to  attribute 
greater  activity  in  this  respect  to  hypogene  solutions  than  is  gen- 
eral among  economic  mineralogists.  This  is  due  mainly  to  the 
fact  that  microscopic  examination  of  mineral  deposits  shows  a  con- 
tinuous series  of  replacements  to  have  occurred  and  often  there 
is  no  reason  for  considering  those  concerned  with  the  deposition 
of  the  late  silver  minerals  as  different  in  kind  from  the  earlier 
processes. 

6.  Certain  of  the  silver  minerals,  notably  argentite  and  pyrar- 
gyrite,  are  rapidly  blackened  on  polished  surfaces  by  an  unpro- 
tected electric  arc,  a  feature  that  has  been  found  of  use  in  iden- 
tification. 

7.  The  complicated  structure  of  stromeyerite  seen  in  some 
specimens  is  the  result  of  a  mixture  of  stromeyerite  and  chal- 
cocite.  Stromeyerite  is  believed  to  be  a.  definite  double  salt  of 
silver  sulphide  and  copper  sulphide.  Chalcocite  probably  is  able 
to  hold  some  silver  sulphide  in  solid  solution  but  the  limit  of  solu- 
bility has  not  been  worked  out. 

8.  The  peculiar  concentric  structure  seen  in  cobalt-nickel  min- 
erals is  believed  to  be  due  to  the  habit  of  the  early  mineral, 
smaltite,  to  replace  calcite  in  the  form  of  concentric  shells.  The 
later  minerals  replacing  the  remainder  of  the  calcite  inside  the 
shells  complete  the  structure.  The  spaces  between  the  shells  are 
sometimes  filled  (by  replacement  of  calcite)  with  the  early  min- 
eral smaltite,  less  often  by  the  later  mineral  niccolite. 

9.  Micro-chemical  tests  applied  on  fragments  secured  from 
the  polished  surfaces  are  considered  more  useful  in  identifying 
silver  minerals  than  the  etching  or  tarnishing  methods. 

10.  The  order  of  deposition  of  the  minerals  in  silver  deposits 
is  outlined  below.  Important  deviation  from  this  order  has  not 
been  observed. 


340  F.  N.  GUILD. 

I.     Silver-lead-zinc  series. 

(i)  Pyrite,  (2)  sphalerite,  (3)  tetrahedrite,  (4)  ga- 
lena, (5)  ruby  silver,  polybasite,  stephanite,  etc.,  (6) 
native  silver.  Chalcopyrite  in  this  series  appears  quite 
typically  in  two  generations,  one  replacing  any  one  or 
all  of  the  sulphides  earlier  than  galena,  the  other  later 
than  galena  and  the  rich  silver  minerals. 
II.     Copper-silver  series. 

(i)  Pyrite,  (2)  chalcopyrite,  (3)  bornite,  (4)  chalco- 
cite,   stromeyerite  and  argentite,    (5)    silver.     Galena 
when   present   is   probably   betw^een   chalcopyrite    and 
bornite. 
in.     Cobalt-silver  series. 

(i)   Smaltite  (chloanthite),   (2)   niccolite  (briethaupt- 
ite),  (3)  argentite,  (4)  silver  and  bismuth, 

X.    ACKNOWLEDGMENTS. 

The  preparation  of  the  foregoing  paper  was  made  possible  by  a 
sabbatical  leave  granted  the  author  by  the  University  of  Arizona. 
He  desires  therefore,  first  of  all,  to  express  his  appreciation  for  the 
courtesy. 

The  investigations  were  conducted  in  the  laboratories  of  the 
Department  of  Geology  of  Leland  Stanford  Junior  University, 
and  the  author  w-ishes  to  acknowledge  his  indebtedness  to  Pro- 
fessor C.  F.  Tolman,  Jr.,  and  Professor  A.  F.  Rogers,  whose 
helpful  suggestions  and  useful  criticisms  are  greatly  appreciated, 
helpful    advice   and    useful    criticisms    are    greatly    appreciated. 

Acknowled'gment  is  also  due  to  Professor  Bailey  Willis  for  a 
critical  reading  of  the  manuscript  as  well  as  for  valuable  sug- 
gestions. 


342  F.  N.  GUllJJ 


EXPLANATION  OF  PLATE  X. 

Fig.  a.  (X21.)  Pj-rite  (py)  crossed  and  bordered  by  chalcopyrit'e  (cp) 
with  later  sphalerite    {si),  galena  (gn)   and  stephanite   (st).     Sandon,  B.  C. 

Fig.  B.  (X6i.)  Arsenopyrite  (as)  and  pyrite  (py)  replaced  by  galena 
(gn).    Proustite  (ps)  in  galena  areas. 

Fig.  C.  (X6i.)  Sphalerite  (si),  argentiferous  tetrahedrite  (td)  and  ga- 
lena (gn).  Sphalerite  dotted  with  chalcopyrite.  Silver  King  mine,  near 
Globe,  Arizona. 

Fig.  D.  (X21.)  Argentiferous  tetrahedrite  (td)  and  chalcopyrite  (cp) 
replaced  by  galena  (gn).    Sphalerite  (si).    Silver  King,  Arizona. 


Plate  X. 


Economic  Geology.    Vol.  XII. 


Fig.  a. 


Fig.  B. 


-jr.'  ^.  -Tivt;.-...,'-      .  ■••'•■    V-  ■ 
■«^-^     ■■■■  sj.:  ■■  f-Pi-^^r  ^ 


\ 


Fig.  C. 


Fig.  D. 


Plate  XL 


Economic  Geology.     Vol.  XII. 


Fig.  a. 


^ 


N 


y 


i> 

\ 

5" 


Fig.  B. 


^> 


-  V  A^.  •"\  ^'  .Y 


Fir..  C. 


Fig.  D. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  343 


EXPLANATION  OF  PLATE  XL 

Fig  A  (X  "Oi  )  Galena  containing  .35  per  cent,  silver  showing  spots  of 
tet!rhetite^an"d  other  silver  minerals.  Etched  with  nitr.c  aad.  R.m.m. 
Montana. 

Fig.  B.  (X336.)  Galena  (gn)  showing  spots  of  proustite  (ps).  Tono- 
pah,  Nevada. 

Fig.  C  (X257.)  Pyrargyrite  following  cleavage  directions  in  galena. 
Etched  with  nitric  acid.     Sandon,  B.  C. 

Fig.  D.  (X30i.)  Sphalerite  (./)  replaced  by  tetrahedrite  (fd).  Silver- 
smith  mine,  Sandon,  B.  C. 


344  F.  N.  GUILD. 


EXPLANATION  OF  PLATE  XIL 

Fig.  a.  (X57-)  Stromeyerite  (so)  replacing  tetrahedrife  (td)  in  a 
complex  system  of  veinlets.     Silver  King,  Arizona. 

Fig.  B.  (X  I70-)  Stromeyerite  (so)  replacing  tetrahedrite  (td)  in  cracks 
and  along  the  borders  of  quartz  gangue.     Colorado. 

Fig.  C.  (X223.)  Replacement  of  chalcopyrite  (cp)  by  bornite  (bn)  with 
veinlet  of  stromeyerite  (so)  crossing  each.  Native  silver  (s)  is  the  last 
mineral  to  form.  Large  area  of  stromeyerite  shows  complex  surface  due 
to  excess  of  the  chalcocite  molecule.     Silver  King,  Arizona. 

Fig.  D.  (X6i.)  Replacement  of  bornite  (bii)  by  stromej'erite  (so)  in 
coarse  craphic  design.     Silver  King,  Arizona. 


Plate  XII. 


Economic  Geology.     Vol.  Xli. 


SO 


Fig.  B. 


% 


Fig.  C. 


Fig.  D. 


Plate  XIII. 

r 


Economic  Geology.    Vol.  XI 


Fig.  a. 


Fig.  B. 


Fig.  C. 


Fig.  D. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  345 


EXPLANATION  OF  PLATE  XIIL 

Fig.  a.  (X267.)  Replacement  of  chalcopyrite  (cp)  by  stromeyerite  (so), 
leaving  residual  grains  grouped  in  graphic  design.     Mt.  Lyell,  Tasmania. 

Fig.  B.  (X98.)  Stromeyerite  etched  with  potassium  cyanide  solution  to 
bring  out  structure.  Leaf-shaped  masses  are  pure  stromeyerite.  The  black 
portion  more  strongly  attacked  is  chalcocite.     Silver  King,  Arizona. 

Fig.  C.  (X288.)  Pure  stromeyerite  areas  (so)  grading  into  chalcocite 
(cc).  The  border  between  the  two  shows  structure  of  Fig.  B,  due  to  mix- 
ture of  the  two.     Unknown  locality,  Arizona. 

Fig.  D.  (X288.)  Same  as  Fig.  C  but  not  etched.  Residual  mass  of 
tetrahedrite  also  shown  (td).     Arizona. 


346  F.  N.  GUILD. 


EXPLANATION  OF  PLATE  XIV. 

Fig.  a.  (X603.)  So-called  intergrowth  of  galena  (light)  and  stromey- 
erite  (dark).     Silver  King,  Arizona. 

Fig.  B.  (X262.)  Sphalerite  (si)  crossed  by  a  veinlet  of  galena  (gn) 
and  proustite   (ps).     Schemnitz,  Hungary, 

Fig.  C.  {y^77-)  Proustite  filling  cavities  between  euhedral  quartz  crys- 
tals iq)  and  altering  to  native  silver  {s).     Durango,  Mexico. 

Fig.  D.  (X8i.)  Pyrargyrite  (pr)  and  proustite  {ps)  filling  cavities  in 
quartz  gangue  (</).  Pyrargyrite  slightly  tarnished  by  electric  light,  thus 
showing  contrast  otherwise  almost  imperceptible.    Tonopah,  Nevada. 


Plate  XIV. 


Economic  Geology.     Vol.  XII. 


Fig.  a. 


Fig.  B. 


Fig.  C. 


Fig.  D. 


Plate  XV. 


Economic  Geology     Vol.  XII. 


Fig.  a. 


Fig.  B. 


■■r.-n?TV~; 


Wi^V^- 


Fig.  C. 


1-lG.    D. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  347 


EXPLANATION  OF  PLATE  XV. 

Fig.  a.  (X74-)  Pyrargyrite  ipr),  proustite  {ps)  and  tetrahedrite  {td) 
in  cavities  in  quartz  gangue.  This  is  part  filling  and  part  replacement. 
Tonopah,  Nevada. 

Fig.  B.  (X88i.)  An  area  of  so-called  intergrowth  of  proustite  {ps)  and 
galena  {gn)  occurring  as  a  residual  mass  in  pyrargyrite  {pr).  Pyrargyrite 
was  blackened  by  action  of  electric  light  employed  in  photographing.  Schem- 
nitz,  Hungary. 

Fig.  C.  (X47-)  A  late  generation  of  iron  disulfid  (marcasite,  ma)  in  a 
background  of  proustite  and  pyrargyrite  in  intimate  mixture.  Cobalt,  On- 
tario, 

Fig.  D.  (X  iP-)  The  sulfids,  pyrite  {py),  sphalerite  {si)  and  stephanite 
{st)  replacing  fragments  in  a  brecciated  gangue  consisting  mostly  of  quartz. 
Sandon,  B.  C. 


348  F.  N.  GUILD. 


EXPLANATION  OF  PLATE  XVL 

Fig.  A.  (X94-)  Sphalerite  (^/),  galena  (£fn)  and  stephanite  (j/)-  Schem- 
nitz,  Hungary. 

Fig.  B.  (X48.)  Sphalerite  (si)  with  dots  of  chalcopyrite  (cp)  replaced 
by  veinlet  of  galena  (gn).  Small  patches  of  polybasite  (pb)  later  replaces 
galena.     Guanacevi,  Mexico. 

Fig.  C.  (X4I-)  Argentite  (ar)  replaced  by  a  late  generation  of  iron 
disulfid,  pyrite  or  marcasite  (py).     Freiberg,  Saxony. 

Fig.  D.  (X265.)  Argentite  (ar)  replaced  around  borders  by  a  fringe  of 
late  generation  chalcopj-rite  (cp).  Argentite  is  blackened  by  the  electric 
light.     Butte,  Montana. 


Plate  XVI. 


Economic  Geology.    Vol.  XII. 


Fig.  a. 


Fig.  B. 


,V^' 

m 

^S^Mm 

«p||^^n^^|H^^^9^^H 

-  .ir^VjJir*                                                                ^^^^^^H^^^l 

^ms  '*J9m 

Fig.  C. 


Fig.  D. 


Plate  XVII. 


Economic  Geology.    Vol.  XII. 


?^-  V'4-    Jew  £  ^'  •  -'d'^*-' 


Fig.  a. 


Fiu.  B. 


:^^i.> 


I 


Fig.  C. 


Fig.  D. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  349 


EXPLANATION  OF  PLATE  XVII. 

Fig.   a.     (X48.)     Veinlet   of   argentite    {ar)    in   smaltite    {sm).    Cobalt, 
Ontario. 

Fig.  B.     (X94-)     Veinlet  of  native  silver   {s)    in  argentite   (or).    Butte, 
Montana. 

Fig.   C.     (X223.)     Native  silver    {s)    in  chalcocite    (cc).    This  structure 
frequently  results  from  the  breakdown  of  stromeyerite.     Silver  King,  Arizona. 

Fig.  D.     (X9-)     Native  silver    {s)   in  veinlet  of  tetrahedrite   {td)   which 
crosses  smaltite   {sm).    Cobalt,  Ontario. 


35^  F.  N.  GUILD. 


EXPLANATION  OF  PLATE  XVIIL 

Fig.  A.  (X  102.)  Veinlet  of  niccolite  (tic)  in  smaltite  (sm),  followed  up 
by  native  silver  (s).    Cobalt,  Ontario. 

Fig.  B.  (X48-)  Native  silver  (s)  replacing  smaltite  (sm).  Euhedral 
crystals  of  smaltite  embedded  in  silver  form  a  characteristic  feature  of 
these  ores.     Cobalt,  Ontario. 

Fig.  C.  (X224.)  The  replacement  of  bornite  (bn)  by  native  silver  (s). 
Cobalt,  Ontario. 

Fig.  D.  (X380.)  Replacement  of  bornite  (dark)  by  native  silver  (light) 
approaching  the  so-called  graphic  intergrowth.     Cobalt,  Ontario. 


Plate  XVIII. 


Economic  Geology.    Vol.  XII. 


WF, 


^v^ 


)*,* 


J  '  .  ,  <r.  \ 


r 


iW 


V'.-.*#^^'^- 


Fig.  a. 


Fig.  B. 


••  -    r  V 


y  .^  - 


V-f^ 


^'-.Vs^ 


^  \  >^ 


.'i^y-^'i^ 


.  »>• 


Fig.  C. 


Fig.  D. 


Plate  XIX. 


Economic  Geology.  Vol.  Xll. 


Fig.  a. 


Fig.  B. 


Fig.  C. 


Fig.  D. 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  351 


EXPLANATION  OF  PLATE  XIX. 

Fig.  a.     (X9-)     Dendritic  silver  and  smaltite  in  calcite.     Cobalt',  Ontario. 

Fig.  B.  (X48.)  The  same  as  Fig.  A  under  higher  magnification.  A  cen- 
tral portion  of  native  silver  {s)  is  seen  with  a  thin  shell  of  smaltite  {sm). 
Cobalt,  Ontario. 

Fig.  C.  (X21.)  Native  silver  {s)  in  siderite  {sd)  altered  to  cerargyrife 
{ce).    Stonewall  Jackson  mine,  near  Prescott,  Arizona. 

Fig.  D.  (Xs6.)  So-called  huntilite  (/im)  with  rim  of  native  arsenic  {as) 
in  a  carbonate  gangue.     Silver  Islet,  Lake  Superior. 


3  52  F.  N.  GUILD. 


EXPLANATION  OF  PLATE  XX. 

Fig.  A.  (X223.)  Micro-structure  of  so-called  brongniardite,  showing  a 
breakdown  into  pyrargyrite  (dark  lines)  and  unknown  constituents.  Pyrar- 
gyrite  lines  show  etching  by  electric  light.     Chococomete,  Bolivia. 

Fig.  B.  (X74-)  Native  bismuth  (bi)  with  silver  occupying  the  center  of 
a  series  of  concentric  shells  made  up  of  niccolite  {71c)  followed  by  an  inter- 
growth  of  niccolite,  breifhauptite,  smaltite  and  chloanthite  {nc  and  sm)  and 
then  smaltite-chloanthite  (stii).     Cobalt,  Ontario. 

Fig.  C.  (X9-)  Concentric  shells  of  smaltite  (chloanthite)  in  calcite 
gangue.     Cobalt,  Ontario. 

Fig.  D.  (X  II-)  Shells  of  smaltite  replacing  calcite  gangue.  Cobalt, 
Ontario. 


Plate  XX. 


Economic  Geology.    Vol.  XII. 


\   \V 


Fig.  a. 


Fig.  B. 


£i 

»..  »i». isSSe.  'j»^.^;^ 

-  ■4- 

^■■^^^^^.ff^BlHB^^^^^^I^^B 

V 

SUSi^^^^bsSsLi 

% 

■*■■*. 

^ 

r>--v^  >-., 

Lf-:^ 

S^-' 

H 

^            -*•  '         :/  .■■■  ',■■/■ 

ir^- 

Fig.  C. 


Fig.  D. 


Plate  XXI. 


Economic  Geology.    Vol.  XII. 


■\  ."- 


•■/ 


mm 


>     V 


*4i>  (^n 


^s  -i^ki^:^M 


Fig.  /.  2> 


"5^:;? 


Fig.  /.  C, 


Fig.  /.  /^ 


FlG.^  ^ 


MICROSCOPIC  STUDY  OF  SILVER  ORES.  353 


EXPLANATION  OF  PLATE  XXL 

Fig.  a.     (X53-)     Same  as  Fig.  C  (Plate  XX.),  showing  individual  shells 
under  higher  power.     Cobalt,  Ontario. 

Fig.  B.     (X48.)     Smaltite   (sm)   replacing  calcite   (ca)   in  peculiar  radiat- 
ing forms.     Cobalt,  Ontario. 

Fig.  C.     (X288.)     Gangue  minerals.     Calcite   (ca)   replacing  quartz  (qu). 
Sandon,  B.  C. 

Fig.  D.     (X74-)     Galena  (gn)  replacing  calcite  gangue  (ca)  along  cleav- 
age directions.     Broken  Hill,  Australia. 


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